Move runtime js files down to lib/runtime/dart

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.
 
-dart_library.library('dart_runtime/dart', null, /* Imports */[
-  'dart_runtime/_classes',
-  'dart_runtime/_errors',
-  'dart_runtime/_generators',
-  'dart_runtime/_operations',
-  'dart_runtime/_rtti',
-  'dart_runtime/_types',
+dart_library.library('dart/_runtime', null, /* Imports */[
+  'dart/_classes',
+  'dart/_errors',
+  'dart/_generators',
+  'dart/_operations',
+  'dart/_rtti',
+  'dart/_types',
 ], /* Lazy Imports */[
   'dart/_js_helper'
 ], function(exports, classes, errors, generators, operations, rtti, types,
     _js_helper) {
   'use strict';
 
   function _export(value) {
     if (value) return value;
     console.log("Re-exporting null field: " + name);
     throw "Bad export";
@@ skipping 90 matching lines @@
   ]);
 
   // From rtti
   exportFrom(rtti, [
     'fn',
     'realRuntimeType',
     'runtimeType',
   ]);
 
 });