Disallow underscores in named formal parameters.

tests/language/parameter_initializer6_negative_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.
+
+// It is a compile-time error if a named formal parameter begins with an '_'
+
+class Foo {
+  num _y;
+  Foo.optional_private([this._y = 77]) {}
+}
+
+main() {
+  var obj;
+  obj = new Foo.optional_private(_y: 222);
+  Expect.equals(222, obj._y);
+
+  obj = new Foo.optional_private();
+  Expect.equals(77, obj._y);
+}