fixes #157, renaming local library identifiers if needed.

lib/runtime/dart_runtime.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.
 
 var dart, _js_helper, _js_primitives;
 (function (dart) {
   'use strict';
 
   let defineProperty = Object.defineProperty;
   let getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
@@ skipping 948 matching lines @@
 
   // TODO(jmesserly): right now this is a sentinel. It should be a type object
   // of some sort, assuming we keep around `dynamic` at runtime.
   dart.dynamic = { toString() { return 'dynamic'; } };
   dart.void = { toString() { return 'void'; } };
   dart.bottom = { toString() { return 'bottom'; } };
 
   dart.global = window || global;
   dart.JsSymbol = Symbol;
 
+  function import_(value) {
+    if (!value) throw 'missing required module';
+    return value;
+  }
+  dart.import = import_;
+  
+  function lazyImport(value) {
+    return defineLibrary(value, {});
+  }
+  dart.lazyImport = lazyImport;
+
+  function defineLibrary(value, defaultValue) {
+    return value ? value : defaultValue;
+  }
+  dart.defineLibrary = defineLibrary;
+
   // TODO(jmesserly): hack to bootstrap the SDK
   _js_helper = _js_helper || {};
   _js_helper.checkNum = notNull;
 
   _js_primitives = _js_primitives || {};
   _js_primitives.printString = (s) => console.log(s);
 
 })(dart || (dart = {}));