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

pkg/compiler/lib/src/mirrors/dart2js_library_mirror.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.mirrors;
 
 
 class Dart2JsLibraryMirror
     extends Dart2JsElementMirror
     with ObjectMirrorMixin, ContainerMixin
@@ skipping 216 matching lines @@
   int get offset => _span.begin;
 
   int get length => _span.end - _span.begin;
 
   String get text => _script.text.substring(_span.begin, _span.end);
 
   Uri get sourceUri => _script.resourceUri;
 
   String get sourceText => _script.text;
 }