dart2js: add classNameFromIsCheckProperty to compiler builtins.

sdk/lib/_internal/compiler/js_lib/js_rti.dart

Index: sdk/lib/_internal/compiler/js_lib/js_rti.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_rti.dart b/sdk/lib/_internal/compiler/js_lib/js_rti.dart
index ed75bc859e3dfd5d28cc6d45eb954a7e29c87ca7..2d462802a897e6b88d7c5839c08ddff4078520aa 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_rti.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_rti.dart
@@ -271,14 +271,9 @@ bool checkSubtype(Object object, String isField, List checks, String asField) {
 ///
 /// In minified mode, uses the unminified names if available.
 String computeTypeName(String isField, List arguments) {
-  // Shorten the field name to the class name and append the textual
+  // Extract the class name from the is field and append the textual
   // representation of the type arguments.
-  // TODO(floitsch): change this to:
-  // String className = JS_BUILTIN('depends:none;effects:none;returns:String',
-  //                               JsBuiltin.classNameFroIsCheckProperty,
-  //                               isField);
-  int prefixLength = JS_OPERATOR_IS_PREFIX().length;
-  return Primitives.formatType(isField.substring(prefixLength, isField.length),
+  return Primitives.formatType(classNameFromIsCheckProperty(isField),
                                arguments);
 }