Implementation of modified scoping for initializing formals.

tests/language/initializing_formal_scope_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.
 //
 // DartOptions=--initializing-formal-access
 
 import "package:expect/expect.dart";
 
+// Duplicate definition checks for `this.x` will check the scopes associated
+// with the constructor, not all enclosing scopes; so this is not a conflict.
+var x;
+
 class A {
-  var x, y;
-  A(this.x) : y = (() => x);
+  var x;
+  A(this.x) {
+    // In the body the field is in scope, not the initializing formal;
+    // so we can use the setter.
+    x += 1;
+  }
 }
 
 main() {
   A a = new A(2);
-  a.x = 3;
   Expect.equals(a.x, 3);
-  Expect.equals(a.y(), 2);
 }