Disable argument type optimization for named parameters.

dart/tests/language/named_parameter_regression_test.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.
+
+// A regression test for dart2js bug 6015.
+
+class Fisk {
+  foo({b, a: true}) {
+    if (b == null) return;
+    throw 'broken';
+  }
+
+  bar({a, b: true}) {
+    if (a == null) return;
+    throw 'broken';
+  }
+}
+
+main() {
+  new Fisk().foo(a: true);
+  new Fisk().bar(b: true);
+}