Return null from stringValue and call slowToString() instead of toString().

dart/frog/leg/ssa/closure.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.
 
 class ClosureFieldElement extends Element {
   ClosureFieldElement(SourceString name, ClassElement enclosing)
       : super(name, ElementKind.FIELD, enclosing);
 
   bool isInstanceMember() => true;
   bool isAssignable() => false;
@@ skipping 223 matching lines @@
     for (Element element in scopeVariables) {
       if (capturedVariableMapping.containsKey(element)) {
         if (box == null) {
           // TODO(floitsch): construct better box names.
           SourceString boxName = new SourceString("box${boxCounter++}");
           box = new Element(boxName,
                             ElementKind.VARIABLE,
                             currentFunctionElement);
         }
         // TODO(floitsch): construct better boxed names.
-        String elementName = element.name.toString();
+        String elementName = element.name.slowToString();
         // We are currently using the name in an HForeign which could replace
         // "$X" with something else.
         String escaped = elementName.replaceAll("\$", "_");
         SourceString boxedName = new SourceString("${escaped}_${boxCounter++}");
         Element boxed = new Element(boxedName, ElementKind.FIELD, box);
         scopeMapping[element] = boxed;
         capturedVariableMapping[element] = boxed;
       }
     }
     if (!scopeMapping.isEmpty()) {
@@ skipping 141 matching lines @@
     }
   }
 
   visitTryStatement(TryStatement node) {
     // TODO(ngeoffray): implement finer grain state.
     inTryCatchOrFinally = true;
     node.visitChildren(this);
     inTryCatchOrFinally = false;
   }
 }