Normalize type masks to use the least upper instantiated subclass/type.

pkg/compiler/lib/src/universe/function_set.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library universe.function_set;
 
 import '../common/names.dart' show
     Identifiers,
     Selectors;
 import '../compiler.dart' show
@@ skipping 280 matching lines @@
     assert(classWorld.hasAnyStrictSubclass(classWorld.objectClass));
     if (_mask != null) return _mask;
     return _mask = new TypeMask.unionOf(functions
         .expand((element) {
           ClassElement cls = element.enclosingClass;
           return [cls]..addAll(classWorld.mixinUsesOf(cls));
         })
         .map((cls) {
           if (classWorld.backend.isNullImplementation(cls)) {
             return const TypeMask.empty();
+          } else if (classWorld.isInstantiated(cls.declaration)) {
+            return new TypeMask.nonNullSubclass(cls.declaration, classWorld);
           } else {
-            return new TypeMask.nonNullSubclass(cls.declaration, classWorld);
+            // TODO(johnniwinther): Avoid the need for this case.
+            return const TypeMask.empty();
           }
         }),
         classWorld);
   }
 }