fixes #145, optional params+initializing formals+private fields

test/codegen/temps.dart

 // Copyright (c) 2015, 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 FormalCollision {
   // These shouldn't collide (see issue #136)
   int _x, __x;
   // This shouldn't generate a keyword as an identifier.
   Function _function;
   FormalCollision(this._x, this.__x, this._function);
 }
 
+class OptionalArg {
+  int _opt;
+  OptionalArg([this._opt = 123]);
+  OptionalArg.named({this._opt: 456});
+}
+
 main() {
   print(new FormalCollision(1, 2, (x) => x));
+  print(new OptionalArg()._opt);
+  print(new OptionalArg.named()._opt);
 }