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

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

Index: pkg/compiler/lib/src/tree/dartstring.dart
diff --git a/pkg/compiler/lib/src/tree/dartstring.dart b/pkg/compiler/lib/src/tree/dartstring.dart
index 72d0b0c060ca0dd37ae01b62ea91cf78bdc36486..7ada1fa2fdfd80a8d129a374c27c992af5db35ff 100644
--- a/pkg/compiler/lib/src/tree/dartstring.dart
+++ b/pkg/compiler/lib/src/tree/dartstring.dart
@@ -2,7 +2,8 @@
 // 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 'dart:collection';
+import '../util/characters.dart';
 
 /**
  * The [DartString] type represents a Dart string value as a sequence of Unicode
@@ -248,4 +249,3 @@ class StringEscapeIterator implements Iterator<int>{
     return true;
   }
 }
-