Introduce FieldParameterElement.

tests/language/src/FieldParameterTest.dart

+// Copyright (c) 2012, 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.
+// Dart test program for testing setting/getting of instance fields.
+
+class A {
+  var x = 4;
+  A(this.x);
+  A.named([this.x]);
+  A.named2([this.x = 2]);
+  A.named3();
+}
+
+class B extends A {
+  B(x) : super(x + 10);
+  B.named_() : super.named();
+  B.named(x) : super.named(x + 10);
+  B.named2_() : super.named2();
+  B.named2(x) : super.named2(x + 10);
+  B.named3() : super.named3();
+}
+
+main() {
+  Expect.equals(0, new A(0).x);
+  Expect.equals(null, new A.named().x);
+  Expect.equals(1, new A.named(1).x);
+  Expect.equals(2, new A.named2().x);
+  Expect.equals(3, new A.named2(3).x);
+  Expect.equals(4, new A.named3().x);
+
+  Expect.equals(10, new B(0).x);
+  Expect.equals(null, new B.named_().x);
+  Expect.equals(11, new B.named(1).x);
+  Expect.equals(2, new B.named2_().x);
+  Expect.equals(13, new B.named2(3).x);
+  Expect.equals(4, new B.named3().x);
+}