Refine the type of a local after it has been used as the receiver of a call.

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

Index: sdk/lib/_internal/compiler/implementation/world.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/world.dart	(revision 26339)
+++ sdk/lib/_internal/compiler/implementation/world.dart	(working copy)
@@ -47,6 +47,11 @@
     return _typesImplementedBySubclasses[cls.declaration];
   }
 
+  bool hasSubclasses(ClassElement cls) {
+    Set<ClassElement> subclasses = compiler.world.subclassesOf(cls);
+    return subclasses != null && !subclasses.isEmpty;
+  }
+
   World(Compiler compiler)
       : allFunctions = new FunctionSet(compiler),
         this.compiler = compiler;