Remove unnecessary imports to tree.dart

pkg/compiler/lib/src/string_validator.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.
 
 // Check the validity of string literals.
 
 library stringvalidator;
 
 import 'dart:collection';
 
 import 'common.dart';
 import 'tokens/token.dart' show Token;
-import 'tree/tree.dart';
+import 'tree/nodes.dart' show StringQuoting;
+import 'tree/dartstring.dart' show DartString;
 import 'util/characters.dart';
 
 class StringValidator {
   final DiagnosticReporter reporter;
 
   StringValidator(this.reporter);
 
   DartString validateInterpolationPart(Token token, StringQuoting quoting,
       {bool isFirst: false, bool isLast: false}) {
     String source = token.value;
@@ skipping 195 matching lines @@
       return null;
     }
     // String literal successfully validated.
     if (quoting.raw || !containsEscape) {
       // A string without escapes could just as well have been raw.
       return new DartString.rawString(string, length);
     }
     return new DartString.escapedString(string, length);
   }
 }