Remove unnecessary imports to tree.dart

pkg/compiler/lib/src/constants/constant_system.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;
 
 import '../dart_types.dart';
 import '../compiler.dart' show Compiler;
 import '../resolution/operators.dart';
-import '../tree/tree.dart' show DartString;
+import '../tree/dartstring.dart' show DartString;
 import 'values.dart';
 
 abstract class Operation {
   String get name;
 }
 
 abstract class UnaryOperation extends Operation {
   /** Returns [:null:] if it was unable to fold the operation. */
   ConstantValue fold(ConstantValue constant);
 }
@@ skipping 116 matching lines @@
         return greaterEqual;
       case BinaryOperatorKind.EQ:
         return equal;
       case BinaryOperatorKind.IF_NULL:
         return ifNull;
       default:
         return null;
     }
   }
 }