Check for duplicate library names

lib/runtime/dart/isolate.js

-var isolate = dart.defineLibrary(isolate, {});
-var core = dart.import(core);
-var _isolate_helper = dart.lazyImport(_isolate_helper);
-var async = dart.import(async);
-(function(exports, core, _isolate_helper, async) {
+dart.library('dart/isolate', null, /* Imports */[
+  'dart/core',
+  'dart/async'
+], /* Lazy imports */[
+  'dart/_isolate_helper'
+], function(exports, core, async, _isolate_helper) {
   'use strict';
   class Capability extends core.Object {
     static new() {
       return new _isolate_helper.CapabilityImpl();
     }
   }
   dart.setSignature(Capability, {
     constructors: () => ({new: [Capability, []]})
   });
   class IsolateSpawnException extends core.Object {
@@ skipping 236 matching lines @@
     constructors: () => ({_RemoteStackTrace: [_RemoteStackTrace, [core.String]]})
   });
   // Exports:
   exports.Capability = Capability;
   exports.IsolateSpawnException = IsolateSpawnException;
   exports.Isolate = Isolate;
   exports.SendPort = SendPort;
   exports.ReceivePort = ReceivePort;
   exports.RawReceivePort = RawReceivePort;
   exports.RemoteError = RemoteError;
-})(isolate, core, _isolate_helper, async);
+});