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

dart/frog/leg/native_emitter.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 NativeEmitter {
 
   Compiler compiler;
   bool addedDynamicFunction = false;
 
   NativeEmitter(this.compiler);
@@ skipping 112 matching lines @@
     buffer.add('$dynamicName = ');
     buffer.add(buildDynamicFunctionCode());
     buffer.add('\n');
     buffer.add(buildDynamicMetadataCode());
     buffer.add('\n');
   }
 
   void generateNativeClass(ClassElement classElement, StringBuffer buffer) {
     addDynamicFunctionIfNecessary(buffer);
     assert(classElement.backendMembers.isEmpty());
-    String nativeName = classElement.nativeName.stringValue;
+    String nativeName = classElement.nativeName.slowToString();
     nativeName = nativeName.substring(2, nativeName.length - 1);
     for (Element member in classElement.members) {
       if (member.isInstanceMember()) {
         String memberName = compiler.namer.getName(member);
         if (member.kind === ElementKind.FUNCTION
             || member.kind === ElementKind.GENERATIVE_CONSTRUCTOR_BODY
             || member.kind === ElementKind.GETTER
             || member.kind === ElementKind.SETTER) {
           String codeBlock = compiler.universe.generatedCode[member];
           if (codeBlock == null) continue;
@@ skipping 46 matching lines @@
     // method does not have subclasses.
     buffer.add('  if (Object.getPrototypeOf(this).hasOwnProperty(');
     buffer.add("'$invocationName')) {\n");
     buffer.add('    return this.${compiler.namer.getName(member)}($arguments)');
     buffer.add('\n  }\n');
     buffer.add('  return Object.prototype.$invocationName.call(this');
     buffer.add(arguments == '' ? '' : ', $arguments');
     buffer.add(');');
   }
 }