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

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

Index: pkg/compiler/lib/src/universe/function_set.dart
diff --git a/pkg/compiler/lib/src/universe/function_set.dart b/pkg/compiler/lib/src/universe/function_set.dart
index 72a1813e91bc2fd22f13b005e1e6fb6901329c06..9f13c1ea2293b8a2eecbd0fddca6ebbb9be8596b 100644
--- a/pkg/compiler/lib/src/universe/function_set.dart
+++ b/pkg/compiler/lib/src/universe/function_set.dart
@@ -298,8 +298,11 @@ class FullFunctionSetQuery implements FunctionSetQuery {
         .map((cls) {
           if (classWorld.backend.isNullImplementation(cls)) {
             return const TypeMask.empty();
-          } else {
+          } else if (classWorld.isInstantiated(cls.declaration)) {
             return new TypeMask.nonNullSubclass(cls.declaration, classWorld);
+          } else {
+            // TODO(johnniwinther): Avoid the need for this case.
+            return const TypeMask.empty();
           }
         }),
         classWorld);