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

tests/compiler/dart2js/setlet_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.
 
 import "package:expect/expect.dart";
 import 'dart:collection';
-import 'package:compiler/src/util/setlet.dart';
+import 'package:compiler/implementation/util/setlet.dart';
 
 main() {
   for (int i = 1; i <= 32; i++) {
     test(i);
   }
 
   testAllLikeSet();
 }
 
 test(int size) {
@@ skipping 131 matching lines @@
     Expect.isFalse(true, '$operationName: unexpected result type');
   }
 }
 
 checkModifications(operationName, setReceiver, setletReceiver) {
   Expect.equals(setReceiver.length, setletReceiver.length,
       '$operationName: same post-operation receiver length');
   Expect.listEquals(setReceiver.toList(), setletReceiver.toList(),
       '$operationName: same post-operation receiver contents');
 }