Introduce FieldParameterElement.

tests/language/src/CompileTimeConstantGTest.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();
+}
+
+class B extends A {
+  const B(x) : super(x + 10);
+  const B.named_() : super.named();
+  const B.named(x) : super.named(x + 10);
+  const B.named2_() : super.named2();
+  const B.named2(x) : super.named2(x + 10);
+  const B.named3() : super.named3();
+}
+
+final b1 = const B(0);
+final b2 = const B.named_();
+final b3 = const B.named(1);
+final b4 = const B.named2_();
+final b5 = const B.named2(3);
+final b6 = const B.named3();
+
+main() {
+  Expect.equals(10, b1.x);
+  Expect.equals(null, b2.x);
+  Expect.equals(11, b3.x);
+  Expect.equals(2, b4.x);
+  Expect.equals(13, b5.x);
+  Expect.equals(4, b6.x);
+}