Fix issue with dart2dart translation of parameters (collision risk).

tests/language/regress_10996_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 "regress_10996_lib.dart" as lib;
+
+foo(a, [b]) {
+  return a + b + lib.a + lib.b;
+}
+
+bar(c, {d}) {
+  return c + d + lib.c + lib.d;
+}
+
+main() {
+  Expect.equals(1 + 2 + 3 + 4, foo(1, 2));
+  Expect.equals(7 + 8 + 3 + 4, foo(7, 8));
+  Expect.equals(3 + 4 + 5 + 6, bar(3, d: 4));
+  Expect.equals(7 + 8 + 5 + 6, bar(7, d: 8));
+}