fixes #221, initial sync*, async, async* implementation

lib/runtime/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;
 (function (dart_library) {
   'use strict';
@@ skipping 85 matching lines @@
 
   function library(name, defaultValue, imports, lazyImports, loader) {
     return libraries[name] =
       new LibraryLoader(name, defaultValue, imports, lazyImports, loader);
   }
   dart_library.library = library;
 
   function import_(libraryName) {
     bootstrap();
     let loader = libraries[libraryName];
+    if (loader == null) {
+      dart_utils.throwError('library not found: ' + libraryName);
+    }
     return loader.load();
   }
   dart_library.import = import_;
 
   function start(libraryName) {
     let library = import_(libraryName);
     let _isolate_helper = import_('dart/_isolate_helper');
     _isolate_helper.startRootIsolate(library.main, []);
   }
   dart_library.start = start;
@@ skipping 14 matching lines @@
     // TODO(vsm): DOM facades?
     // See: https://github.com/dart-lang/dev_compiler/issues/173
     NodeList.prototype.get = function(i) { return this[i]; };
     NamedNodeMap.prototype.get = function(i) { return this[i]; };
     DOMTokenList.prototype.get = function(i) { return this[i]; };
     HTMLCollection.prototype.get = function(i) { return this[i]; };
 
   }
 
 })(dart_library || (dart_library = {}));