diff --git a/src/stream.js b/src/stream.js
index 5f099168..2fef58ef 100644
--- a/src/stream.js
+++ b/src/stream.js
@@ -1,6 +1,24 @@
-import util from './util';
+const NodeReadableStream = typeof window === 'undefined' && require('stream').Readable;
 
-const NodeReadableStream = util.getNodeStream();
+const isIE11 = typeof navigator !== 'undefined' && !!navigator.userAgent.match(/Trident\/7\.0.*rv:([0-9.]+).*\).*Gecko$/);
+
+/**
+ * Check whether data is a Stream
+ * @param {Any} input  data to check
+ * @returns {Boolean}
+ */
+function isStream(input) {
+  return ReadableStream.prototype.isPrototypeOf(input);
+}
+
+/**
+ * Check whether data is a Uint8Array
+ * @param {Any} input  data to check
+ * @returns {Boolean}
+ */
+function isUint8Array(input) {
+  return Uint8Array.prototype.isPrototypeOf(input);
+}
 
 /**
  * Convert data to Stream
@@ -8,7 +26,7 @@ const NodeReadableStream = util.getNodeStream();
  * @returns {ReadableStream} Converted data
  */
 function toStream(input) {
-  if (util.isStream(input)) {
+  if (isStream(input)) {
     return input;
   }
   return new ReadableStream({
@@ -20,17 +38,60 @@ function toStream(input) {
 }
 
 /**
- * Concat a list of Streams
- * @param {Array<ReadableStream|Uint8array|String>} list  Array of Uint8Arrays/Strings/Streams to concatenate
- * @returns {ReadableStream} Concatenated array
+ * Concat a list of Uint8Arrays, Strings or Streams
+ * The caller should not mix Uint8Arrays with Strings, but may mix Streams with non-Streams.
+ * @param {Array<Uint8array|String|ReadableStream>} Array of Uint8Arrays/Strings/Streams to concatenate
+ * @returns {Uint8array|String|ReadableStream} Concatenated array
  */
 function concat(list) {
+  if (list.some(isStream)) {
+    return concatStream(list);
+  }
+  if (typeof list[0] === 'string') {
+    return list.join('');
+  }
+  return concatUint8Array(list);
+}
+
+/**
+ * Concat Uint8Arrays
+ * @param {Array<Uint8array>} Array of Uint8Arrays to concatenate
+ * @returns {Uint8array} Concatenated array
+ */
+function concatUint8Array(arrays) {
+  if (arrays.length === 1) return arrays[0];
+
+  let totalLength = 0;
+  for (let i = 0; i < arrays.length; i++) {
+    if (!isUint8Array(arrays[i])) {
+      throw new Error('concatUint8Array: Data must be in the form of a Uint8Array');
+    }
+
+    totalLength += arrays[i].length;
+  }
+
+  const result = new Uint8Array(totalLength);
+  let pos = 0;
+  arrays.forEach(function (element) {
+    result.set(element, pos);
+    pos += element.length;
+  });
+
+  return result;
+}
+
+/**
+ * Concat a list of Streams
+ * @param {Array<ReadableStream|Uint8array|String>} list  Array of Uint8Arrays/Strings/Streams to concatenate
+ * @returns {ReadableStream} Concatenated list
+ */
+function concatStream(list) {
   list = list.map(toStream);
   const transform = transformWithCancel(async function(reason) {
-    await Promise.all(transforms.map(array => cancel(array, reason)));
+    await Promise.all(transforms.map(stream => cancel(stream, reason)));
   });
   let prev = Promise.resolve();
-  const transforms = list.map((array, i) => transformPair(array, (readable, writable) => {
+  const transforms = list.map((stream, i) => transformPair(stream, (readable, writable) => {
     prev = prev.then(() => pipe(readable, transform.writable, {
       preventClose: i !== list.length - 1
     }));
@@ -65,9 +126,7 @@ function getWriter(input) {
  * @returns {Promise<undefined>} Promise indicating when piping has finished (input stream closed or errored)
  */
 async function pipe(input, target, options) {
-  if (!util.isStream(input)) {
-    input = toStream(input);
-  }
+  input = toStream(input);
   try {
     if (input[externalBuffer]) {
       const writer = target.getWriter();
@@ -77,10 +136,8 @@ async function pipe(input, target, options) {
       }
       writer.releaseLock();
     }
-    return await input.pipeTo(target, options).catch(function() {});
-  } catch(e) {
-    util.print_debug_error(e);
-  }
+    return await input.pipeTo(target, options);
+  } catch(e) {}
 }
 
 /**
@@ -144,7 +201,7 @@ function transformWithCancel(cancel) {
  * @returns {ReadableStream|Uint8array|String}
  */
 function transform(input, process = () => undefined, finish = () => undefined) {
-  if (util.isStream(input)) {
+  if (isStream(input)) {
     return transformRaw(input, {
       async transform(value, controller) {
         try {
@@ -166,7 +223,7 @@ function transform(input, process = () => undefined, finish = () => undefined) {
   }
   const result1 = process(input);
   const result2 = finish();
-  if (result1 !== undefined && result2 !== undefined) return util.concat([result1, result2]);
+  if (result1 !== undefined && result2 !== undefined) return concat([result1, result2]);
   return result1 !== undefined ? result1 : result2;
 }
 
@@ -229,7 +286,7 @@ function parse(input, fn) {
  * @returns {Array<ReadableStream|Uint8array|String>} array containing two copies of input
  */
 function tee(input) {
-  if (util.isStream(input)) {
+  if (isStream(input)) {
     const teed = input.tee();
     teed[0][externalBuffer] = teed[1][externalBuffer] = input[externalBuffer];
     return teed;
@@ -245,7 +302,7 @@ function tee(input) {
  * @returns {ReadableStream|Uint8array|String} cloned input
  */
 function clone(input) {
-  if (util.isStream(input)) {
+  if (isStream(input)) {
     const teed = tee(input);
     overwrite(input, teed[0]);
     return teed[1];
@@ -262,7 +319,7 @@ function clone(input) {
  * @returns {ReadableStream|Uint8array|String} cloned input
  */
 function passiveClone(input) {
-  if (util.isStream(input)) {
+  if (isStream(input)) {
     return new ReadableStream({
       start(controller) {
         const transformed = transformPair(input, async (readable, writable) => {
@@ -319,7 +376,7 @@ function overwrite(input, clone) {
  * @returns {ReadableStream|Uint8array|String} clone
  */
 function slice(input, begin=0, end=Infinity) {
-  if (util.isStream(input)) {
+  if (isStream(input)) {
     if (begin >= 0 && end >= 0) {
       let bytesRead = 0;
       return transformRaw(input, {
@@ -340,12 +397,12 @@ function slice(input, begin=0, end=Infinity) {
       return transform(input, value => {
         if (value.length >= -begin) lastBytes = [value];
         else lastBytes.push(value);
-      }, () => slice(util.concat(lastBytes), begin, end));
+      }, () => slice(concat(lastBytes), begin, end));
     }
     if (begin === 0 && end < 0) {
       let lastBytes;
       return transform(input, value => {
-        const returnValue = lastBytes ? util.concat([lastBytes, value]) : value;
+        const returnValue = lastBytes ? concat([lastBytes, value]) : value;
         if (returnValue.length >= -end) {
           lastBytes = slice(returnValue, end);
           return slice(returnValue, begin, end);
@@ -354,27 +411,26 @@ function slice(input, begin=0, end=Infinity) {
         }
       });
     }
-    // TODO: Don't read entire stream into memory here.
-    util.print_debug_error(`stream.slice(input, ${begin}, ${end}) not implemented efficiently.`);
+    console.warn(`stream.slice(input, ${begin}, ${end}) not implemented efficiently.`);
     return fromAsync(async () => slice(await readToEnd(input), begin, end));
   }
   if (input[externalBuffer]) {
-    input = util.concat(input[externalBuffer].concat([input]));
+    input = concat(input[externalBuffer].concat([input]));
   }
-  if (util.isUint8Array(input) && !util.isIE11) { // IE11 subarray is buggy
+  if (isUint8Array(input) && !isIE11) { // IE11 subarray is buggy
     return input.subarray(begin, end);
   }
   return input.slice(begin, end);
 }
 
 /**
- * Read a stream to the end and return its contents, concatenated by the concat function (defaults to util.concat).
+ * Read a stream to the end and return its contents, concatenated by the concat function (defaults to concat).
  * @param {ReadableStream|Uint8array|String} input
  * @param {Function} concat
  * @returns {Uint8array|String|Any} the return value of concat()
  */
 async function readToEnd(input, concat) {
-  if (util.isStream(input)) {
+  if (isStream(input)) {
     return getReader(input).readToEnd(concat);
   }
   return input;
@@ -387,7 +443,7 @@ async function readToEnd(input, concat) {
  * @returns {Promise<Any>} indicates when the stream has been canceled
  */
 async function cancel(input, reason) {
-  if (util.isStream(input)) {
+  if (isStream(input)) {
     return input.cancel(reason);
   }
 }
@@ -490,7 +546,7 @@ if (NodeReadableStream) {
 }
 
 
-export default { toStream, concat, getReader, getWriter, pipe, transformRaw, transform, transformPair, parse, clone, passiveClone, slice, readToEnd, cancel, nodeToWeb, webToNode, fromAsync };
+export default { isStream, isUint8Array, toStream, concatUint8Array, concatStream, concat, getReader, getWriter, pipe, transformRaw, transform, transformPair, parse, clone, passiveClone, slice, readToEnd, cancel, nodeToWeb, webToNode, fromAsync };
 
 
 const doneReadingSet = new WeakSet();
@@ -500,7 +556,7 @@ function Reader(input) {
   if (input[externalBuffer]) {
     this[externalBuffer] = input[externalBuffer].slice();
   }
-  if (util.isStream(input)) {
+  if (isStream(input)) {
     const reader = input.getReader();
     this._read = reader.read.bind(reader);
     this._releaseLock = reader.releaseLock.bind(reader);
@@ -555,12 +611,12 @@ Reader.prototype.readLine = async function() {
   while (!returnVal) {
     const { done, value } = await this.read();
     if (done) {
-      if (buffer.length) return util.concat(buffer);
+      if (buffer.length) return concat(buffer);
       return;
     }
     const lineEndIndex = value.indexOf('\n') + 1;
     if (lineEndIndex) {
-      returnVal = util.concat(buffer.concat(value.substr(0, lineEndIndex)));
+      returnVal = concat(buffer.concat(value.substr(0, lineEndIndex)));
       buffer = [];
     }
     if (lineEndIndex !== value.length) {
@@ -593,13 +649,13 @@ Reader.prototype.readBytes = async function(length) {
   while (true) {
     const { done, value } = await this.read();
     if (done) {
-      if (buffer.length) return util.concat(buffer);
+      if (buffer.length) return concat(buffer);
       return;
     }
     buffer.push(value);
     bufferLength += value.length;
     if (bufferLength >= length) {
-      const bufferConcat = util.concat(buffer);
+      const bufferConcat = concat(buffer);
       this.unshift(slice(bufferConcat, length));
       return slice(bufferConcat, 0, length);
     }
@@ -628,16 +684,16 @@ Reader.prototype.unshift = function(...values) {
 };
 
 /**
- * Read the stream to the end and return its contents, concatenated by the concat function (defaults to util.concat).
- * @param {Function} concat
- * @returns {Uint8array|String|Any} the return value of concat()
+ * Read the stream to the end and return its contents, concatenated by the join function (defaults to concat).
+ * @param {Function} join
+ * @returns {Uint8array|String|Any} the return value of join()
  */
-Reader.prototype.readToEnd = async function(concat=util.concat) {
+Reader.prototype.readToEnd = async function(join=concat) {
   const result = [];
   while (true) {
     const { done, value } = await this.read();
     if (done) break;
     result.push(value);
   }
-  return concat(result);
+  return join(result);
 };
diff --git a/src/util.js b/src/util.js
index 0954c4bc..fec6db77 100644
--- a/src/util.js
+++ b/src/util.js
@@ -31,11 +31,7 @@ import util from './util'; // re-import module to access util functions
 import b64 from './encoding/base64';
 import stream from './stream';
 
-const isIE11 = typeof navigator !== 'undefined' && !!navigator.userAgent.match(/Trident\/7\.0.*rv:([0-9.]+).*\).*Gecko$/);
-
 export default {
-  isIE11,
-
   isString: function(data) {
     return typeof data === 'string' || String.prototype.isPrototypeOf(data);
   },
@@ -44,13 +40,9 @@ export default {
     return Array.prototype.isPrototypeOf(data);
   },
 
-  isUint8Array: function(data) {
-    return Uint8Array.prototype.isPrototypeOf(data);
-  },
+  isUint8Array: stream.isUint8Array,
 
-  isStream: function(data) {
-    return ReadableStream.prototype.isPrototypeOf(data);
-  },
+  isStream: stream.isStream,
 
   /**
    * Get transferable objects to pass buffers with zero copy (similar to "pass by reference" in C++)
@@ -346,57 +338,14 @@ export default {
    * @param {Array<Uint8array|String|ReadableStream>} Array of Uint8Arrays/Strings/Streams to concatenate
    * @returns {Uint8array|String|ReadableStream} Concatenated array
    */
-  concat: function (list) {
-    if (list.some(util.isStream)) {
-      return stream.concat(list);
-    }
-    if (util.isString(list[0])) {
-      return list.join('');
-    }
-    return util.concatUint8Array(list);
-  },
+  concat: stream.concat,
 
   /**
    * Concat Uint8Arrays
    * @param {Array<Uint8array>} Array of Uint8Arrays to concatenate
    * @returns {Uint8array} Concatenated array
    */
-  concatUint8Array: function (arrays) {
-    if (arrays.length === 1) return arrays[0];
-
-    let totalLength = 0;
-    for (let i = 0; i < arrays.length; i++) {
-      if (!util.isUint8Array(arrays[i])) {
-        throw new Error('concatUint8Array: Data must be in the form of a Uint8Array');
-      }
-
-      totalLength += arrays[i].length;
-    }
-
-    const result = new Uint8Array(totalLength);
-    let pos = 0;
-    arrays.forEach(function (element) {
-      result.set(element, pos);
-      pos += element.length;
-    });
-
-    return result;
-  },
-
-  /**
-   * Deep copy Uint8Array
-   * @param {Uint8Array} Array to copy
-   * @returns {Uint8Array} new Uint8Array
-   */
-  copyUint8Array: function (array) {
-    if (!util.isUint8Array(array)) {
-      throw new Error('Data must be in the form of a Uint8Array');
-    }
-
-    const copy = new Uint8Array(array.length);
-    copy.set(array);
-    return copy;
-  },
+  concatUint8Array: stream.concatUint8Array,
 
   /**
    * Check Uint8Array equality