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

tests/compiler/dart2js/deferred_not_in_main_test.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.
 
 // Test of the graph segmentation algorithm used by deferred loading
 // to determine which elements can be deferred and which libraries
 // much be included in the initial download (loaded eagerly).
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import 'memory_source_file_helper.dart';
 import "dart:async";
 
-import 'package:compiler/src/dart2jslib.dart'
+import 'package:compiler/implementation/dart2jslib.dart'
        as dart2js;
 
 class FakeOutputStream<T> extends EventSink<T> {
   void add(T event) {}
   void addError(T event, [StackTrace stackTrace]) {}
   void close() {}
 }
 
 void main() {
   Uri script = currentDirectory.resolveUri(Platform.script);
@@ skipping 54 matching lines @@
 void foo1() {
   lib1.loadLibrary().then((_) => lib2.foo2());
 }
 """,
   "lib2.dart":"""
 library lib2;
 
 void foo2() {}
 """,
 };