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

dart/frog/tests/leg/src/ResolverTest.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.
 
 #import("../../../leg/leg.dart");
 #import("../../../leg/elements/elements.dart");
 #import("../../../leg/tree/tree.dart");
 #import("../../../leg/util/util.dart");
 #import("mock_compiler.dart");
 #import("parser_helper.dart");
@@ skipping 410 matching lines @@
   Expect.isTrue(element is SynthesizedConstructorElement);
 }
 
 testTopLevelFields() {
   MockCompiler compiler = new MockCompiler();
   compiler.parseScript("int a;");
   VariableElement element = compiler.mainApp.find(buildSourceString("a"));
   Expect.equals(ElementKind.FIELD, element.kind);
   VariableDefinitions node = element.variables.parseNode(compiler);
   Identifier typeName = node.type.typeName;
-  Expect.equals(typeName.source.stringValue, 'int');
+  Expect.equals(typeName.source.slowToString(), 'int');
 
   compiler.parseScript("var b, c;");
   VariableElement bElement = compiler.mainApp.find(buildSourceString("b"));
   VariableElement cElement = compiler.mainApp.find(buildSourceString("c"));
   Expect.equals(ElementKind.FIELD, bElement.kind);
   Expect.equals(ElementKind.FIELD, cElement.kind);
   Expect.isTrue(bElement != cElement);
 
   VariableDefinitions bNode = bElement.variables.parseNode(compiler);
   VariableDefinitions cNode = cElement.variables.parseNode(compiler);
@@ skipping 158 matching lines @@
 length(Link link) => link.isEmpty() ? 0 : length(link.tail) + 1;
 
 at(Link link, int index) => (index == 0) ? link.head : at(link.tail, index - 1);
 
 List<String> asSortedStrings(Link link) {
   List<String> result = <String>[];
   for (; !link.isEmpty(); link = link.tail) result.add(link.head.toString());
   result.sort((s1, s2) => s1.compareTo(s2));
   return result;
 }