Store named parameters in sorted lists instead of using maps.

testcases/input/named_parameters.dart

+// Copyright (c) 2016, 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.
+class Superclass {
+  foo({alpha, beta}) {}
+  bar({beta, alpha}) {}
+}
+
+class Subclass extends Superclass {
+  foo({beta, alpha}) {}
+  bar({alpha, beta}) {}
+}
+
+topLevelNamed(beta, alpha, {gamma, delta}) {}
+topLevelOptional(beta, alpha, [gamma, delta]) {}
+
+main() {
+  new Subclass().foo(beta: 1, alpha: 2);
+  new Subclass().foo(alpha: 1, beta: 2);
+  topLevelNamed(1, 2, gamma: 3, delta: 4);
+  topLevelNamed(1, 2, delta: 3, gamma: 4);
+}