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

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of ssa;
 
 /**
  * A special element for the extra parameter taken by intercepted
  * methods. We need to override [Element.computeType] because our
  * optimizers may look at its declared type.
@@ skipping 3362 matching lines @@
       } else if (Elements.isGrowableListConstructorCall(
                     originalElement, send, compiler)) {
         isListConstructor = true;
         HType inferred =
             new HType.inferredForNode(currentElement, send, compiler);
         return inferred.isUnknown() ? backend.extendableArrayType : inferred;
       } else if (element.isGenerativeConstructor()) {
         ClassElement cls = element.getEnclosingClass();
         return new HType.nonNullExact(cls.thisType, compiler);
       } else {
-        return new HType.inferredTypeForElement(originalElement, compiler);
+        return new HType.inferredReturnTypeForElement(
+            originalElement, compiler);
       }
     }
 
     Element constructor = elements[send];
     Selector selector = elements.getSelector(send);
     if (constructor.isForwardingConstructor) {
       compiler.unimplemented('forwarded constructor in named mixin application',
                              element: constructor.getEnclosingClass());
     }
     if (compiler.enqueuer.resolution.getCachedElements(constructor) == null) {
@@ skipping 2008 matching lines @@
           new HSubGraphBlockInformation(elseBranch.graph));
 
     HBasicBlock conditionStartBlock = conditionBranch.block;
     conditionStartBlock.setBlockFlow(info, joinBlock);
     SubGraph conditionGraph = conditionBranch.graph;
     HIf branch = conditionGraph.end.last;
     assert(branch is HIf);
     branch.blockInformation = conditionStartBlock.blockFlow;
   }
 }