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

tests/try/poi/diff_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 element diff.
 library trydart.diff_test;
 
 import 'dart:async' show
     Future;
 
 import 'package:expect/expect.dart' show
     Expect;
 
 import 'package:async_helper/async_helper.dart' show
     asyncTest;
 
-import 'package:compiler/src/dart2jslib.dart' show
+import 'package:compiler/implementation/dart2jslib.dart' show
     Compiler,
     Script;
 
-import 'package:compiler/src/source_file.dart' show
+import 'package:compiler/implementation/source_file.dart' show
     StringSourceFile;
 
-import 'package:compiler/src/elements/elements.dart' show
+import 'package:compiler/implementation/elements/elements.dart' show
     Element,
     LibraryElement;
 
 import 'package:dart2js_incremental/diff.dart' show
     Difference,
     computeDifference;
 
 import '../../compiler/dart2js/compiler_helper.dart' show
     MockCompiler,
     compilerFor;
@@ skipping 79 matching lines @@
         expectedBeforeName, elementNameOrNull(difference.before));
     Expect.stringEquals(expectedAfterName, elementNameOrNull(difference.after));
     print(difference);
   }
   Expect.isFalse(iterator.moveNext());
 }
 
 void main() {
   asyncTest(() => Future.forEach(TEST_DATA, testData));
 }