Test implicit this on fields.

dart/tests/language/src/ImplicitThisTest.dart

+// Copyright (c) 2011, 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.
+
+interface Interface {
+  final x;
+}
+
+// The abstract keyword is implicit if a class declares an abstract
+// member. This class does not declare any abstract members, so it
+// must be marked abstract because it doesn't implement the x getter.
+abstract class Abstract implements Interface {
+  String toString() => x.toString();
+}
+
+// This class does not implement "x" either, but it is not marked
+// abstract.
+class SubAbstract1 extends Abstract { } /// 01: static type error
+
+// This class is implicitly abstract as it declares an abstract getter
+// method.
+class SubAbstract2 extends Abstract {
+  abstract get x();
+}
+
+// This class does not implement "x" either, but it is not marked
+// abstract.
+class SubSubAbstract2 extends SubAbstract2 { } /// 04: static type error
+
+class Concrete extends Abstract {
+  get x() => 7;
+}
+
+class SubConcrete extends Concrete {
+  final x;
+  SubConcrete(this.x);
+}
+
+void main() {
+  var x = new Abstract(); /// 02: static type error
+  var y = new SubAbstract1(); /// 01: continued
+  var z = new SubAbstract2(); /// 03: static type error
+  var a = new SubSubAbstract2(); /// 04: continued
+  Expect.equals(x, x); /// 02: continued
+  Expect.equals('7', new Concrete().toString());
+  Expect.equals('42', new SubConcrete(42).toString());
+  Expect.equals('7', new SubConcrete(new Concrete()).toString());
+}