Reapply 'Add runtimeType() to Object which returns canonicalized instances of Type'.

lib/compiler/implementation/world.dart

Index: lib/compiler/implementation/world.dart
diff --git a/lib/compiler/implementation/world.dart b/lib/compiler/implementation/world.dart
index 0e6410a37301324a3abb98dbcf030834bae144e8..4ff3eb388806e7dc0014b3cbaced68b2a823bdd8 100644
--- a/lib/compiler/implementation/world.dart
+++ b/lib/compiler/implementation/world.dart
@@ -72,7 +72,9 @@ class World {
     }
   }
 
-  bool needsRti(ClassElement cls) => classesNeedingRti.contains(cls);
+  bool needsRti(ClassElement cls) {
+    return classesNeedingRti.contains(cls) || compiler.enabledRuntimeType;
+  }
 
   void registerRtiDependency(Element element, Element dependency) {
     // We're not dealing with typedef for now.