Disallow underscores in named formal parameters.

tests/language/parameter_initializer_test.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
 class ParameterInitializerTest {
 
   static testMain() {
     var obj = new Foo.untyped(1);
     Expect.equals(1, obj.x);
 
     obj = new Foo.supertype(9);
     Expect.equals(9, obj.x);
 
     obj = new Foo.subtype(7);
     Expect.equals(7, obj.x);
 
     obj = new Foo.optional(x: 111);
     Expect.equals(111, obj.x);
 
     obj = new Foo.optional();
     Expect.equals(5, obj.x);
 
-    obj = new Foo.optional_private(_y: 222);
-    Expect.equals(222, obj._y);
-
-    obj = new Foo.optional_private();
-    Expect.equals(77, obj._y);
-
     obj = new Foo(1);
     Expect.equals(2, obj.x);
 
     obj = new SubFoo(42);
     Expect.equals(1, obj.x);
 
     obj = new SubSubFoo(42);
     Expect.equals(1, obj.x);
   }
 }
 
 class Foo {
   Foo(num this.x) {
     // Reference to x must resolve to the field.
     x++;
     Expect.equals(this.x, x);
   }
 
   Foo.untyped(this.x) {}
   Foo.supertype(Object this.x) {}
   Foo.subtype(int this.x) {}
   Foo.optional([this.x = 5]) {}
-  Foo.optional_private([this._y = 77]) {}
 
   num x;
-  num _y;
 }
 
 class SubFoo extends Foo {
   SubFoo(num y) : super(y), x_ = 0 {
     // Subfoo.setter of x has been invoked in the Foo constructor.
     Expect.equals(x, 1);
     Expect.equals(x_, 1);
 
     // The super.x will resolved to the field in Foo.
     Expect.equals(super.x, y);
@@ skipping 17 matching lines @@
     Expect.equals(x_, 1);
 
     // There is no way to get to the field in Foo.
     Expect.equals(super.x, 1);
   }
 }
 
 main() {
   ParameterInitializerTest.testMain();
 }