Support virtual fields in DDC without requiring the @virtual annotation.

tests/language_strong/field_override6_test.dart

+// Copyright (c) 2017, 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.
+
+import "package:expect/expect.dart";
+import "package:meta/meta.dart";
+
+abstract class A {
+  get x1;
+  get x3;
+  @virtual
+  var x5 = 1;
+  @virtual
+  var x6 = 1;
+  @virtual
+  var x8 = 1;
+  @virtual
+  var x9 = 1;
+
+  @virtual
+  var xB = 1;
+  @virtual
+  var xC = 1;
+  @virtual
+  var xE = 1;
+  @virtual
+  var xF = 1;
+}
+
+abstract class B extends A {
+  @virtual
+  var x1 = 2;
+  @virtual
+  var x3 = 2;
+
+  get x5;
+  get x6;
+
+  set x8(v);
+  set x9(v);
+
+  get xB;
+  set xB(v) {
+    super.xB = v;
+  }
+
+  get xC;
+  set xC(v) {
+    super.xC = v;
+  }
+
+  get xE => super.xE;
+  set xE(v);
+  get xF => super.xF;
+  set xF(v);
+}
+
+class C extends B {
+  var x1 = 3;
+  get x1_ => 3;
+
+  get x2 => super.x1;
+  get x2_ => 2;
+
+  get x3 => 3;
+  get x3_ => 3;
+
+  get x4 => super.x3;
+  get x4_ => 2;
+
+  get x5;
+  get x5_ => 1;
+
+  get x6;
+  get x6_ => 1;
+
+  get x7 => super.x6;
+  get x7_ => 1;
+
+  get x8;
+  get x8_ => 1;
+
+  get x9;
+  get x9_ => 1;
+
+  get xA => super.x9;
+  get xA_ => 1;
+
+  get xB;
+  get xB_ => 1;
+
+  get xC;
+  get xC_ => 1;
+
+  get xD => super.xC;
+  get xD_ => 1;
+
+  get xE;
+  get xE_ => 1;
+
+  get xF;
+  get xF_ => 1;
+
+  get xG => super.xF;
+  get xG_ => 1;
+}
+
+main() {
+  var c = new C();
+  Expect.equals(c.x1_, c.x1);
+  Expect.equals(c.x2_, c.x2);
+  Expect.equals(c.x3_, c.x3);
+  Expect.equals(c.x4_, c.x4);
+  Expect.equals(c.x5_, c.x5);
+  Expect.equals(c.x6_, c.x6);
+  Expect.equals(c.x7_, c.x7);
+  Expect.equals(c.x8_, c.x8);
+  Expect.equals(c.x9_, c.x9);
+  Expect.equals(c.xA_, c.xA);
+  Expect.equals(c.xB_, c.xB);
+  Expect.equals(c.xC_, c.xC);
+  Expect.equals(c.xD_, c.xD);
+  Expect.equals(c.xE_, c.xE);
+  Expect.equals(c.xF_, c.xF);
+  Expect.equals(c.xG_, c.xG);
+}