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

dart/frog/leg/typechecker.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 TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name() => "Type checker";
 
   static final bool LOG_FAILURES = false;
 
@@ skipping 40 matching lines @@
 
   String toString() => stringName;
 }
 
 class SimpleType implements Type {
   final SourceString name;
   final Element element;
 
   const SimpleType(SourceString this.name, Element this.element);
 
-  String toString() => name.toString();
+  String toString() => name.slowToString();
 }
 
 class FunctionType implements Type {
   final Element element;
   final Type returnType;
   final Link<Type> parameterTypes;
 
   const FunctionType(Type this.returnType, Link<Type> this.parameterTypes,
                      Element this.element);
 
@@ skipping 594 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     compiler.unimplemented('visitCatchBlock', node: node);
   }
 
   visitTypedef(Typedef node) {
     compiler.unimplemented('visitTypedef', node: node);
   }
 }