Make ddc legacy loader more defensive

pkg/dev_compiler/lib/js/legacy/dart_library.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 file defines the module loader for the dart runtime.
 */
-
-var dart_library =
+var dart_library;
+if (!dart_library) {
+dart_library =
   typeof module != "undefined" && module.exports || {};
 
 (function (dart_library) {
   'use strict';
 
   /** Note that we cannot use dart_utils.throwInternalError from here. */
   function throwLibraryError(message) {
     throw Error(message);
   }
 
@@ skipping 67 matching lines @@
   dart_library.debuggerLibraries = function() {
     var debuggerLibraries = [];
     libraries.forEach(function (value, key, map) {
       debuggerLibraries.push(value.load());
     });
     debuggerLibraries.__proto__ = null;
     return debuggerLibraries;
   };
 
   function library(name, defaultValue, imports, loader) {
-    let result = new LibraryLoader(name, defaultValue, imports, loader);
+    let result = libraries.get(name);
+    if (result) {
+      console.warn('Already loaded ' + name);
+      return result;
+    }
+    result = new LibraryLoader(name, defaultValue, imports, loader);
     libraries.set(name, result);
     return result;
   }
   dart_library.library = library;
 
   function import_(libraryName) {
     bootstrap();
     let loader = libraries.get(libraryName);
     // TODO(vsm): A user might call this directly from JS (as we do in tests).
     // We may want a different error type.
     if (!loader) throwLibraryError('Library not found: ' + libraryName);
     return loader.load();
   }
   dart_library.import = import_;
 
+  var _currentIsolate = false;
+
   function start(moduleName, libraryName) {
     if (libraryName == null) libraryName = moduleName;
     let library = import_(moduleName)[libraryName];
     let dart_sdk = import_('dart_sdk');
-    dart_sdk._isolate_helper.startRootIsolate(library.main, []);
+    if (!_currentIsolate) {
+      // Create isolate and run main.
+      _currentIsolate = true;
+      dart_sdk._isolate_helper.startRootIsolate(library.main, []);
+    } else {
+      // Main isolate is already initialized - just run main.
+      library.main();
+    }
   }
   dart_library.start = start;
 
   let _bootstrapped = false;
   function bootstrap() {
     if (_bootstrapped) return;
     _bootstrapped = true;
 
     // Force import of core.
     var dart_sdk = import_('dart_sdk');
@@ skipping 45 matching lines @@
         window.SourceBufferList = new MediaSource().sourceBuffers.constructor;
       }
     }
 
     // This import is only needed for chrome debugging. We should provide an
     // option to compile without it.
     dart_sdk._debugger.registerDevtoolsFormatter();
   }
 
 })(dart_library);
+}