Remove unnecessary imports to tree.dart

pkg/compiler/lib/src/constants/values.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.constants.values;
 
 import '../common.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart'
     show FieldElement, FunctionElement, PrefixElement;
-import '../tree/tree.dart' hide unparse;
+import '../tree/dartstring.dart';
 import '../util/util.dart' show Hashing;
 
 abstract class ConstantValueVisitor<R, A> {
   const ConstantValueVisitor();
 
   R visitFunction(FunctionConstantValue constant, A arg);
   R visitNull(NullConstantValue constant, A arg);
   R visitInt(IntConstantValue constant, A arg);
   R visitDouble(DoubleConstantValue constant, A arg);
   R visitBool(BoolConstantValue constant, A arg);
@@ skipping 752 matching lines @@
 
   @override
   DartType getType(CoreTypes types) => const DynamicType();
 
   @override
   String toStructuredText() => 'NonConstant';
 
   @override
   String toDartText() => '>>non-constant<<';
 }