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

sdk/lib/_internal/compiler/implementation/types/type_mask.dart

Index: sdk/lib/_internal/compiler/implementation/types/type_mask.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/types/type_mask.dart	(revision 26339)
+++ sdk/lib/_internal/compiler/implementation/types/type_mask.dart	(working copy)
@@ -19,8 +19,7 @@
   factory TypeMask.subclass(DartType base) => new FlatTypeMask.subclass(base);
   factory TypeMask.subtype(DartType base) => new FlatTypeMask.subtype(base);
 
-  factory TypeMask.nonNullEmpty()
-      => new FlatTypeMask.nonNullEmpty();
+  const factory TypeMask.nonNullEmpty() = FlatTypeMask.nonNullEmpty;
   factory TypeMask.nonNullExact(DartType base)
       => new FlatTypeMask.nonNullExact(base);
   factory TypeMask.nonNullSubclass(DartType base)