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

tests/compiler/dart2js/space_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.
 
 import 'dart:io';
-import 'package:compiler/src/dart2js.dart'
+import 'package:compiler/implementation/dart2js.dart'
   as dart2js;
 
 main() {
   Uri currentDirectory = Uri.base;
   Uri script = currentDirectory.resolveUri(Platform.script);
   Uri libraryRoot = script.resolve('../../../sdk/');
   Directory.current = script.resolve("path with spaces").toFilePath();
 
   return dart2js.main(["--library-root=${libraryRoot.toFilePath()}",
                        "--analyze-only",
                        "file with spaces.dart"]);
 }