Cleanup 1: split parts into their own libraries, remove unused imports

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

Index: pkg/compiler/lib/src/tree/unparser.dart
diff --git a/pkg/compiler/lib/src/tree/unparser.dart b/pkg/compiler/lib/src/tree/unparser.dart
index 807ea0514b9ee1d1b0f32471662f24486ac2ffb2..b9e67841c37fed686ad741b209c8aa98dcdd69bd 100644
--- a/pkg/compiler/lib/src/tree/unparser.dart
+++ b/pkg/compiler/lib/src/tree/unparser.dart
@@ -2,7 +2,10 @@
 // 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.
 
-part of tree;
+import '../tokens/token.dart' show BeginGroupToken, Token;
+import '../tokens/token_constants.dart' as Tokens show IDENTIFIER_TOKEN, KEYWORD_TOKEN, PLUS_TOKEN;
+import '../util/util.dart';
+import 'nodes.dart';
 
 String unparse(Node node, {minify: true}) {
   Unparser unparser = new Unparser(minify: minify);