Introduce isAbstractlyInstantiated to avoid exact masks of abstract classes.

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 7aba1204e3a0cc72a2c71fd221d161d0786c027f..0e95ce5288f8e928b41202ec67a6b00b2bf9d23c 100644
--- a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
@@ -481,9 +481,7 @@ class SimpleTypeInferrerVisitor<T>
         });
       }
       if (analyzedElement.isGenerativeConstructor && cls.isAbstract) {
-        if (compiler.closedWorld.isDirectlyInstantiated(cls)) {
-          returnType = types.nonNullExact(cls);
-        } else if (compiler.closedWorld.isIndirectlyInstantiated(cls)) {
+        if (compiler.closedWorld.isInstantiated(cls)) {
           returnType = types.nonNullSubclass(cls);
         } else {
           // TODO(johnniwinther): Avoid analyzing [analyzedElement] in this