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

pkg/compiler/lib/src/closure.dart

Index: pkg/compiler/lib/src/closure.dart
diff --git a/pkg/compiler/lib/src/closure.dart b/pkg/compiler/lib/src/closure.dart
index 00cb34ca1a62dca2496e1f335ff3f9d295c8ab1c..9b9d6033050d4cb6963d0dc2d5f86ff202e90d93 100644
--- a/pkg/compiler/lib/src/closure.dart
+++ b/pkg/compiler/lib/src/closure.dart
@@ -998,6 +998,9 @@ class ClosureTranslator extends Visitor {
     String closureName = computeClosureName(element);
     ClosureClassElement globalizedElement = new ClosureClassElement(
         node, closureName, compiler, element);
+    // Extend [globalizedElement] as an instantiated class in the closed world.
+    compiler.world.registerClass(
+        globalizedElement, isDirectlyInstantiated: true);
     FunctionElement callElement =
         new SynthesizedCallMethodElementX(Identifiers.call,
                                           element,