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

tests/compiler/dart2js/package_root_test.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.
 
 // Test that the compiler can handle imports when package root has not been set.
 
 library dart2js.test.package_root;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_source_file_helper.dart';
 
-import 'package:compiler/src/dart2jslib.dart'
+import 'package:compiler/implementation/dart2jslib.dart'
        show NullSink;
 
 import 'package:compiler/compiler.dart'
        show DiagnosticHandler, Diagnostic;
 
 import 'dart:async';
 
 const MEMORY_SOURCE_FILES = const {
   'main.dart': '''
 
@@ skipping 38 matching lines @@
     Expect.equals("Cannot resolve 'package:foo/foo.dart'. "
                   "Package root has not been set.",
                   errors[0]);
   }));
 }
 
 void main() {
   runCompiler(Uri.parse('memory:main.dart'));
   runCompiler(Uri.parse('package:foo/foo.dart'));
 }