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

pkg/compiler/lib/src/tree/dartstring.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 'dart:collection';
+import '../util/characters.dart';
 
 /**
  * The [DartString] type represents a Dart string value as a sequence of Unicode
  * Scalar Values.
  * After parsing, any valid [LiteralString] will contain a [DartString]
  * representing its content after removing quotes and resolving escapes in
  * its source.
  */
 abstract class DartString extends IterableBase<int> {
   factory DartString.empty() => const LiteralDartString("");
@@ skipping 225 matching lines @@
           value = value * 16 + hexDigitValue(source.current);
         }
         _current = value;
         break;
       default:
         _current = code;
     }
     return true;
   }
 }
-