Tighten check disallowing use of initializing formals (fixes #26855).

tests/language/regress_26855_test.dart

+// Copyright (c) 2016, 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.
+
+void f0(this.x) {}  /// 0: compile-time error
+
+void f1(int g(this.x)) {}  /// 1: compile-time error
+
+void f2(int g(int this.x)) {}  /// 2: compile-time error
+
+class C {
+  C();
+  var x;
+  void f3(int g(this.x)) {}  /// 3: compile-time error
+  C.f4(int g(this.x));  /// 4: compile-time error
+}
+
+main() {
+  f0(null);  /// 0: continued
+  f1(null);  /// 1: continued
+  f2(null);  /// 2: continued
+  C c = new C();
+  c.f3(null);  /// 3: continued
+  new C.f4(null);  /// 4: continued
+}
+