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

tests/compiler/dart2js/backend_dart/dart_backend_test.dart

 // Copyright (c) 2012, 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.
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 import '../mock_compiler.dart';
 import '../mock_libraries.dart';
 import 'package:compiler/compiler.dart';
-import 'package:compiler/implementation/dart2jslib.dart' as leg;
-import 'package:compiler/implementation/dart_backend/dart_backend.dart';
-import 'package:compiler/implementation/elements/elements.dart';
-import 'package:compiler/implementation/tree/tree.dart';
+import 'package:compiler/src/dart2jslib.dart' as leg;
+import 'package:compiler/src/dart_backend/dart_backend.dart';
+import 'package:compiler/src/elements/elements.dart';
+import 'package:compiler/src/tree/tree.dart';
 
 const ioLib = r'''
 library io;
 class Platform {
   static int operatingSystem;
 }
 ''';
 
 const htmlLib = r'''
 library html;
@@ skipping 1075 matching lines @@
   testParametersMinified();
   testDeclarationTypePlaceholders();
   testPlatformLibraryMemberNamesAreFixed();
   testConflictsWithCoreLib();
   testUnresolvedNamedConstructor1();
   testUnresolvedNamedConstructor2();
   testUnresolvedNamedConstructor3();
   testClassAndNamedMixinDeclarations();
 }