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

pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
index 17c53c6a8d3588314396bf725d0da1d5614e4dda..884ca97d42aabd55d8432e1445fa866aafa05133 100644
--- a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
@@ -644,7 +644,17 @@ class SimpleTypeInferrerVisitor<T>
           }
         });
       }
-      returnType = types.nonNullExact(cls);
+      if (analyzedElement.isGenerativeConstructor && cls.isAbstract) {
+        if (compiler.world.isInstantiated(cls)) {
+          returnType = types.nonNullSubclass(cls);

[Siggi Cherem (dart-lang), 2015/11/03 21:01:57]

same here - why not nonNullExact?

[Johnni Winther, 2015/11/06 13:43:23]

Updated to use nonNullExact if [cls] is directly instantiated.

+        } else  {
+          // TODO(johnniwinther): Avoid analyzing [analyzedElement] in this
+          // case; it's never called.
+          returnType = types.nonNullEmpty();
+        }
+      } else {
+        returnType = types.nonNullExact(cls);
+      }
     } else {
       signature.forEachParameter((LocalParameterElement element) {
         locals.update(element, inferrer.typeOfElement(element), node);