Typecheck constructor initializers properly

test/codegen/expect/async/async.js

 var async;
 (function(async) {
   'use strict';
   dart.copyProperties(async, {
     get _hasDocument() {
       return dart.equals(dart.dinvokef(/* Unimplemented unknown name */JS, 'String', 'typeof document'), 'object');
     }
   });
   // Function _invokeErrorHandler: (Function, Object, StackTrace) → dynamic
   function _invokeErrorHandler(errorHandler, error, stackTrace) {
@@ skipping 3532 matching lines @@
           sink._add(inputEvent);
         }
       }
     }
     return _WhereStream;
   });
   let _WhereStream = _WhereStream$(dynamic);
   let _MapStream$ = dart.generic(function(S, T) {
     class _MapStream extends _ForwardingStream$(S, T) {
       _MapStream(source, transform) {
-        this._transform = transform;
+        this._transform = dart.as(transform, _Transformation);
         super._ForwardingStream(source);
       }
       _handleData(inputEvent, sink) {
         let outputEvent = null;
         try {
           outputEvent = dart.as(this._transform(inputEvent), T);
         } catch (e) {
           let s = dart.stackTrace(e);
           _addErrorWithReplacement(sink, e, s);
           return;
@@ skipping 1217 matching lines @@
   async.StreamIterator$ = StreamIterator$;
   async.StreamController = StreamController;
   async.StreamController$ = StreamController$;
   async.Timer = Timer;
   async.AsyncError = AsyncError;
   async.ZoneSpecification = ZoneSpecification;
   async.ZoneDelegate = ZoneDelegate;
   async.Zone = Zone;
   async.runZoned = runZoned;
 })(async || (async = {}));