Report PRIVATE_OPTIONAL_PARAMETER in more cases.

tests/co19/co19-analyzer.status

 # 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.
 
 [ $compiler == dartanalyzer ]
 
 # not clear: may be assignment in the constructor is warning, not error
 Language/05_Variables/05_Variables_A05_t04: fail
 
 # func(final x())
 Language/06_Functions/2_Formal_Parameters/1_Required_Formals_A02_t06: fail
 
 # func(var x())
 Language/06_Functions/2_Formal_Parameters/1_Required_Formals_A02_t07: fail
 
-# void func({var _x}) {}
-Language/06_Functions/2_Formal_Parameters/2_Optional_Formals_A03_t03: fail
-
 # not clear: g([var foo = foo + 10]) is parameter 'foo' in the scope of its own initialzer?
 Language/06_Functions/2_Formal_Parameters_A02_t02: fail
 
 # not clear: null..[1](1)[2](2).foo(3, bar: 4)=5 - it seems that verything before =5 it not assignable
 Language/07_Classes/6_Constructors/1_Generative_Constructors_A04_t15: fail
 
 # not initialized final instance variable, should be static warning
 Language/07_Classes/6_Constructors/1_Generative_Constructors_A09_t01: fail
 
 # invalid argument for constant constructor
@@ skipping 202 matching lines @@
 # co19 issue 438, Static variables are initialized lazily, need not be constants
 Language/11_Expressions/01_Constants_A16_t01: fail, OK
 Language/11_Expressions/01_Constants_A16_t02: fail, OK
 
 # co19 issue 439, it is warning, not error to import two different libraries with the same name
 Language/13_Libraries_and_Scripts/1_Imports_A05_t01: fail, OK
 
 # co19 issue #440, adj strings is not a string interpolation
 Language/13_Libraries_and_Scripts/5_URIs_A01_t24: fail, OK
 Language/13_Libraries_and_Scripts/5_URIs_A01_t25: fail, OK