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";
 
 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.
+    x += 1;
+  }

[Johnni Winther, 2016/06/29 07:22:24]

Also add tests for:

  A(x, this.x)
  A(this.x, x)

and

var x;
class A {
  A(this.x); // This should not clash with the top-level 'x'.
}

[eernst, 2016/06/29 09:28:31]

compiler/dart2js/message_kind_test.dart contains some test cases for duplicate
detection (including `this.x, x`); I added a test there for the opposite
ordering. Added `DUPLICATE_DEFINITION` to the list of multi-message test cases
in message_kind_helper.dart and filed an issue on the fact that we get multiple
messages in this situation (https://github.com/dart-lang/sdk/issues/26788).

Added `var x` at top level in 'initializing_formal_scope_test.dart', to verify
that `ExtensionScope` checks for duplicates using `localLookup` on the extendee
scope rather than `lookup`.

 }
 
 main() {
   A a = new A(2);
-  a.x = 3;
   Expect.equals(a.x, 3);
-  Expect.equals(a.y(), 2);
 }