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

pkg/compiler/lib/src/tree/tree.dart

Index: pkg/compiler/lib/src/tree/tree.dart
diff --git a/pkg/compiler/lib/src/tree/tree.dart b/pkg/compiler/lib/src/tree/tree.dart
deleted file mode 100644
index f69c3a45301b775adff64da607ad847f881d5be8..0000000000000000000000000000000000000000
--- a/pkg/compiler/lib/src/tree/tree.dart
+++ /dev/null
@@ -1,21 +0,0 @@
-// Copyright (c) 2012, 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.
-
-library tree;
-
-import 'dart:collection';
-
-import '../scanner/scannerlib.dart';
-import '../util/util.dart';
-import '../util/characters.dart';
-
-import '../resolution/secret_tree_element.dart'
-    show StoredTreeElementMixin, NullTreeElementMixin;
-
-import '../elements/elements.dart' show MetadataAnnotation;
-
-part 'dartstring.dart';
-part 'nodes.dart';
-part 'prettyprint.dart';
-part 'unparser.dart';