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

tests/compiler/dart2js/unparser_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 'parser_helper.dart';
 import 'mock_compiler.dart';
-import 'package:compiler/src/tree/tree.dart';
+import 'package:compiler/implementation/tree/tree.dart';
 
 testUnparse(String statement) {
   Node node = parseStatement(statement);
   Expect.equals(statement, unparse(node));
 }
 
 testUnparseMember(String member) {
   Node node = parseMember(member);
   Expect.equals(member, unparse(node));
 }
@@ skipping 368 matching lines @@
   testPart();
   testPartOf();
   testCombinators();
   testDeferredImport();
   testRedirectingFactoryConstructors();
   testClassDeclarations();
   testMixinApplications();
   testParameters();
   testSymbolLiterals();
 }