fixes #157, renaming local library identifiers if needed.

lib/runtime/dart/_js_helper.js

-var _js_helper;
-(function(exports) {
+var _js_helper, core, collection, _internal, _foreign_helper, _js_embedded_names, _interceptors, _js_names, async, _isolate_helper;
+(function(exports, core, collection, _internal, _foreign_helper, _js_embedded_names, _interceptors, _js_names, async, _isolate_helper) {
   'use strict';
   class NoSideEffects extends core.Object {
     NoSideEffects() {
     }
   }
   class NoThrows extends core.Object {
     NoThrows() {
     }
   }
   class NoInline extends core.Object {
@@ skipping 4060 matching lines @@
   exports.random64 = random64;
   exports.jsonEncodeNative = jsonEncodeNative;
   exports.getIsolateAffinityTag = getIsolateAffinityTag;
   exports.LoadLibraryFunctionType = LoadLibraryFunctionType;
   exports.DeferredLoadCallback = DeferredLoadCallback;
   exports.loadDeferredLibrary = loadDeferredLibrary;
   exports.MainError = MainError;
   exports.missingMain = missingMain;
   exports.badMain = badMain;
   exports.mainHasTooManyParameters = mainHasTooManyParameters;
-})(_js_helper || (_js_helper = {}));
+})(_js_helper || (_js_helper = {}), core, collection, _internal, _foreign_helper, _js_embedded_names, _interceptors || (_interceptors = {}), _js_names, async, _isolate_helper || (_isolate_helper = {}));