fixes #221, initial sync*, async, async* implementation

lib/runtime/_classes.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /* This library defines the operations that define and manipulate Dart
  * classes.  Included in this are:
  *   - Generics
  *   - Class metadata
  *   - Extension methods
  */
@@ skipping 364 matching lines @@
       let sig = originalSigFn();
       for (let name of methodNames) {
         sig[getExtensionSymbol(name)] = sig[name];
       }
       return sig;
     });
   }
   exports.defineExtensionMembers = defineExtensionMembers;
 
   function canonicalMember(obj, name) {
-    if (obj[_extensionType]) return dartx[name];
+    if (obj != null && obj[_extensionType]) return dartx[name];
     return name;
   }
   exports.canonicalMember = canonicalMember;
 
   /** Sets the type of `obj` to be `type` */
   function setType(obj, type) {
     obj.__proto__ = type.prototype;
     return obj;
   }
 
   /** Sets the element type of a list literal. */
   function list(obj, elementType) {
     return setType(obj, _interceptors.JSArray$(elementType));
   }
   exports.list = list;
 
   function setBaseClass(derived, base) {
     // Link the extension to the type it's extending as a base class.
     derived.prototype.__proto__ = base.prototype;
   }
   exports.setBaseClass = setBaseClass;
 
 });