dart2js: add classNameFromIsCheckProperty to compiler builtins.

pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
index 6a55d08c638de8cd58915dbdac314c2720839d86..a700946fb1617c01a6f1c25fb32a88a685ee9077 100644
--- a/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
@@ -138,6 +138,10 @@ class Emitter implements emitterTask.Emitter {
         return js.js.expressionTemplateYielding(
             typeAccess(_compiler.objectClass));
 
+      case JsBuiltin.classNameFromIsCheckProperty:
+        int isPrefixLength = namer.operatorIsPrefix.length;
+        return js.js.expressionTemplateFor('#.substring($isPrefixLength)');
+
       case JsBuiltin.isFunctionType:
         return _backend.rti.representationGenerator.templateForIsFunctionType;