cleanup to Value - fix for StringEscapesTest

frog/await/checker.dart

 // Copyright (c) 2011, 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.
 
 /**
  * Traverses the entire code of a function to find whether it contains awaits,
  * ensuring they occur only in valid locations. This visitor will *not* recurse
  * inside nested function declarations.
  */
 class AwaitChecker implements TreeVisitor {
@@ skipping 288 matching lines @@
   }
 
   visitThisExpression(ThisExpression node) {
     return false;
   }
 
   visitSuperExpression(SuperExpression node) {
     return false;
   }
 
-  visitNullExpression(NullExpression node) {
+  visitStringInterpExpression(StringInterpExpression node) {
     return false;
   }
 
   visitLiteralExpression(LiteralExpression node) {
     return false;
   }
 
   visitArgumentNode(ArgumentNode node) {
     bool awaitSeen = node.value.visit(this);
     if (awaitSeen) haveAwait.add(node);
@@ skipping 40 matching lines @@
       }
     }
     return awaitSeen;
   }
 
   _visit(node) {
     if (node == null) return false;
     return node.visit(this);
   }
 }