Change how runtime type information is being set in the backend.

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
===================================================================
--- lib/compiler/implementation/lib/js_helper.dart	(revision 11430)
+++ lib/compiler/implementation/lib/js_helper.dart	(working copy)
@@ -960,7 +960,12 @@
 
 getRuntimeTypeInfo(target) {
   if (target === null) return null;
-  return JS('var', @'#.builtin$typeInfo', target);
+  var res = JS('var', @'#.builtin$typeInfo', target);
+  // If the object does not have runtime type information, return an
+  // empty literal, to avoid null checks.
+  // TODO(ngeoffray): Make the object a top-level field to avoid
+  // allocating a new object every single time.
+  return (res == null) ? JS('var', '{}') : res;
 }
 
 /**