Implement CHA through type mask and TypedSelector in the simple type inferrer.

sdk/lib/_internal/compiler/implementation/world.dart

Index: sdk/lib/_internal/compiler/implementation/world.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/world.dart	(revision 19615)
+++ sdk/lib/_internal/compiler/implementation/world.dart	(working copy)
@@ -161,6 +161,10 @@
     return uses != null && !uses.isEmpty;
   }
 
+  bool hasAnySubclass(ClassElement cls) {
+    return subclasses[cls] != null && !subclasses[cls].isEmpty;
+  }
+
   void registerRtiDependency(Element element, Element dependency) {
     // We're not dealing with typedef for now.
     if (!element.isClass() || !dependency.isClass()) return;