Introduce FieldParameterElement.

tests/language/src/CompileTimeConstantFTest.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 {
+  final x = 4;
+  const A(this.x);
+  const A.named([this.x]);
+  const A.named2([this.x = 2]);
+  const A.named3();
+}
+
+final a1 = const A(0);
+final a2 = const A.named();
+final a3 = const A.named(1);
+final a4 = const A.named2();
+final a5 = const A.named2(3);
+final a6 = const A.named3();
+
+main() {
+  Expect.equals(0, a1.x);
+  Expect.equals(null, a2.x);
+  Expect.equals(1, a3.x);
+  Expect.equals(2, a4.x);
+  Expect.equals(3, a5.x);
+  Expect.equals(4, a6.x);
+}