Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/js_emitter/helpers.dart

 // Copyright (c) 2013, 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.
 
 part of dart2js.js_emitter;
 
 ClassElement computeMixinClass(MixinApplicationElement mixinApplication) {
   ClassElement mixin = mixinApplication.mixin;
   while (mixin.isMixinApplication) {
     mixinApplication = mixin;
     mixin = mixinApplication.mixin;
   }
   return mixin;
 }