Added online expansion and compilation of files

being edited in DrRacket (via places)

Added an API to let tools have access to that
  information (and compute more stuff)

Used that to make an online version of Check Syntax
  which led to a separately callable Check Syntax API.

original commit: b972a0940d04f4ff2c659d494805a7d34358da17

collects/typed-scheme/base-env/base-types-extra.rkt

@@ -12,10 +12,15 @@
 
 ;; special type names that are not bound to particular types
 (define-other-types
-  -> case-> U Rec All Opaque Vector
+  #;-> case-> U Rec All Opaque Vector
   Parameterof List List* Class Values Instance Refinement
   pred)
 
+(define-syntax ->
+  (lambda (stx)
+    (raise-syntax-error 'type-check "type name used out of context" stx))) 
+(provide ->)
+
 (provide (rename-out [All ∀]
                      [U Un]
 		     [-> →]