Remove unnecessary imports to tree.dart

pkg/compiler/lib/src/constant_system_dart.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.
 
 library dart2js.constant_system.dart;
 
 import 'compiler.dart' show Compiler;
 import 'constants/constant_system.dart';
 import 'constants/values.dart';
 import 'dart_types.dart';
-import 'tree/tree.dart' show DartString;
+import 'tree/dartstring.dart' show DartString;
 
 const DART_CONSTANT_SYSTEM = const DartConstantSystem();
 
 class BitNotOperation implements UnaryOperation {
   final String name = '~';
   const BitNotOperation();
   ConstantValue fold(ConstantValue constant) {
     if (constant.isInt) {
       IntConstantValue intConstant = constant;
       return DART_CONSTANT_SYSTEM.createInt(~intConstant.primitiveValue);
@@ skipping 436 matching lines @@
   bool isInt(ConstantValue constant) => constant.isInt;
   bool isDouble(ConstantValue constant) => constant.isDouble;
   bool isString(ConstantValue constant) => constant.isString;
   bool isBool(ConstantValue constant) => constant.isBool;
   bool isNull(ConstantValue constant) => constant.isNull;
 
   bool isSubtype(DartTypes types, DartType s, DartType t) {
     return types.isSubtype(s, t);
   }
 }