Remove unnecessary imports to tree.dart

pkg/compiler/lib/src/constants/expressions.dart

 // Copyright (c) 2014, 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.constants.expressions;
 
 import '../common.dart';
 import '../constants/constant_system.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart'
     show
         ConstructorElement,
         FieldElement,
         FunctionElement,
         PrefixElement,
         VariableElement;
 import '../resolution/operators.dart';
-import '../tree/tree.dart' show DartString;
+import '../tree/dartstring.dart' show DartString;
 import '../universe/call_structure.dart' show CallStructure;
 import 'evaluation.dart';
 import 'values.dart';
 
 enum ConstantExpressionKind {
   BINARY,
   BOOL,
   BOOL_FROM_ENVIRONMENT,
   CONCATENATE,
   CONDITIONAL,
@@ skipping 1851 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }