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

tests/compiler/dart2js/inferrer_factory_test.dart

+// Copyright (c) 2013, 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.
+
+import "package:expect/expect.dart";
+import 'compiler_helper.dart';
+
+const String TEST1 = r"""
+class A implements List {
+  factory A() {
+    // Avoid inlining by using try/catch.
+    try {
+      return new List();
+    } catch (e) {
+    }
+  }
+}
+
+main() {
+  new A()[0] = 42;
+}
+""";
+
+main() {
+  String generated = compileAll(TEST1);
+  // Check that we're using the index operator on the object returned
+  // by the A factory.
+  Expect.isTrue(generated.contains('[0] = 42'));
+}
+