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

pkg/compiler/lib/src/types/flat_type_mask.dart

Index: pkg/compiler/lib/src/types/flat_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/flat_type_mask.dart b/pkg/compiler/lib/src/types/flat_type_mask.dart
index ce19f17c8b24f42beb34871eee0e8be65c26cfc4..6516d96eed48994fd46ef7a81a6768e2f1325036 100644
--- a/pkg/compiler/lib/src/types/flat_type_mask.dart
+++ b/pkg/compiler/lib/src/types/flat_type_mask.dart
@@ -546,9 +546,8 @@ class FlatTypeMask implements TypeMask {
    */
   static bool hasConcreteMatch(ClassElement cls,
                                Selector selector,
-                               World world) {
-    assert(invariant(cls,
-        world.compiler.resolverWorld.isInstantiated(cls),
+                               ClassWorld world) {
+    assert(invariant(cls, world.isInstantiated(cls),
         message: '$cls has not been instantiated.'));
     Element element = findMatchIn(cls, selector);
     if (element == null) return false;