fixes #157, renaming local library identifiers if needed.

lib/runtime/dart/async.js

-var async;
-(function(exports) {
+var async = dart.defineLibrary(async, {});
+var core = dart.import(core);
+var _internal = dart.import(_internal);
+var _js_helper = dart.lazyImport(_js_helper);
+var _isolate_helper = dart.lazyImport(_isolate_helper);
+var collection = dart.import(collection);
+(function(exports, core, _internal, _js_helper, _isolate_helper, collection) {
   'use strict';
   // Function _invokeErrorHandler: (Function, Object, StackTrace) → dynamic
   function _invokeErrorHandler(errorHandler, error, stackTrace) {
     if (dart.is(errorHandler, ZoneBinaryCallback)) {
       return dart.dcall(errorHandler, error, stackTrace);
     } else {
       return dart.dcall(errorHandler, error);
     }
   }
   // Function _registerErrorHandler: (Function, Zone) → Function
@@ skipping 5341 matching lines @@
   exports.ErrorCallbackHandler = ErrorCallbackHandler;
   exports.ScheduleMicrotaskHandler = ScheduleMicrotaskHandler;
   exports.CreateTimerHandler = CreateTimerHandler;
   exports.CreatePeriodicTimerHandler = CreatePeriodicTimerHandler;
   exports.PrintHandler = PrintHandler;
   exports.ForkHandler = ForkHandler;
   exports.ZoneSpecification = ZoneSpecification;
   exports.ZoneDelegate = ZoneDelegate;
   exports.Zone = Zone;
   exports.runZoned = runZoned;
-})(async || (async = {}));
+})(async, core, _internal, _js_helper, _isolate_helper, collection);