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

pkg/analyzer2dart/lib/src/dart_backend.dart

 // Copyright (c) 2014, 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.
 
 library analyzer2dart.dart_backend;
 
-import 'package:compiler/src/elements/elements.dart';
-import 'package:compiler/src/dart_backend/dart_backend.dart';
-import 'package:compiler/src/dart2jslib.dart';
+import 'package:compiler/implementation/elements/elements.dart';
+import 'package:compiler/implementation/dart_backend/dart_backend.dart';
+import 'package:compiler/implementation/dart2jslib.dart';
 
 import 'driver.dart';
 import 'converted_world.dart';
 
 void compileToDart(Driver driver, ConvertedWorld convertedWorld) {
   DartOutputter outputter =
       new DartOutputter(new Listener(), driver.outputProvider);
   outputter.assembleProgram(
     libraries: convertedWorld.libraries,
     instantiatedClasses: convertedWorld.instantiatedClasses,
@@ skipping 62 matching lines @@
   @override
   SourceSpan spanFromSpannable(Spannable node) {
     // TODO: implement spanFromSpannable
   }
 
   @override
   withCurrentElement(element, f()) {
     // TODO: implement withCurrentElement
   }
 }