Check for duplicate library names

lib/runtime/dart/_js_helper.js

-var _js_helper = dart.defineLibrary(_js_helper, {});
-var core = dart.import(core);
-var collection = dart.import(collection);
-var _internal = dart.import(_internal);
-var _foreign_helper = dart.import(_foreign_helper);
-var _js_embedded_names = dart.import(_js_embedded_names);
-var _interceptors = dart.lazyImport(_interceptors);
-var _js_names = dart.import(_js_names);
-var async = dart.import(async);
-var _isolate_helper = dart.lazyImport(_isolate_helper);
+dart.defineLibrary('_js_helper');
+dart.import('core');
+dart.import('collection');
+dart.import('_internal');
+dart.import('_foreign_helper');
+dart.import('_js_embedded_names');
+dart.lazyImport('_interceptors');
+dart.import('_js_names');
+dart.import('async');
+dart.lazyImport('_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() {
     }
   }
   dart.setSignature(NoSideEffects, {
     constructors: () => ({NoSideEffects: [NoSideEffects, []]})
   });
   class NoThrows extends core.Object {
@@ skipping 4359 matching lines @@
   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, core, collection, _internal, _foreign_helper, _js_embedded_names, _interceptors, _js_names, async, _isolate_helper);