Implement simple dynamic type check.

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
diff --git a/lib/compiler/implementation/lib/js_helper.dart b/lib/compiler/implementation/lib/js_helper.dart
index 9f5bd4cbd54454bd4870bf8d37b0c16691e70600..9d1adbaf8a9849ba0d3a93775d7482f592e6d6a7 100644
--- a/lib/compiler/implementation/lib/js_helper.dart
+++ b/lib/compiler/implementation/lib/js_helper.dart
@@ -1397,3 +1397,13 @@ class Null {
 }
 
 builtin$get$toString(receiver) => () => builtin$toString$0(receiver);
+
+setRuntimeTypeInfo(target, typeInfo) {
+  // We have to check for null because factories may return null.
+  if (target !== null) JS('var', @'#.builtin$typeInfo = #', target, typeInfo);
+}
+
+getRuntimeTypeInfo(target) {
+  if (target === null) return null;
+  return JS('var', @'#.builtin$typeInfo', target);
+}