Use the inferred return type of the factory instead of the inferred type.

sdk/lib/_internal/compiler/implementation/ssa/builder.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/builder.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(revision 24223)
+++ sdk/lib/_internal/compiler/implementation/ssa/builder.dart	(working copy)
@@ -3380,7 +3380,8 @@
         ClassElement cls = element.getEnclosingClass();
         return new HType.nonNullExact(cls.thisType, compiler);
       } else {
-        return new HType.inferredTypeForElement(originalElement, compiler);
+        return new HType.inferredReturnTypeForElement(
+            originalElement, compiler);
       }
     }