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

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

 // 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.
 
-part of tree;
+import '../tokens/token.dart' show BeginGroupToken, Token;
+import '../util/util.dart';
+import 'nodes.dart';
 
 /**
  * Pretty-prints Node tree in XML-like format.
  *
  * TODO(smok): Add main() to run from command-line to print out tree for given
  * .dart file.
  */
 class PrettyPrinter extends Indentation with Tagging<Node> implements Visitor {
   @override
   void addDefaultParameters(Node node, Map params) {
@@ skipping 454 matching lines @@
   }
 
   visitGotoStatement(GotoStatement node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }