allow overriding abstract fields in NNBD

lib/src/rules/overridden_fields.dart

@@ -125,7 +125,7 @@ class _Visitor extends SimpleAstVisitor<void> {
 
     node.fields.variables.forEach((VariableDeclaration variable) {
       final field = _getOverriddenMember(variable.declaredElement);
-      if (field != null) {
+      if (field != null && !field.isAbstract) {
         rule.reportLint(variable.name);
       }
     });

test/rules/experiments/nnbd/rules/overridden_fields.dart

@@ -0,0 +1,195 @@
+// Copyright (c) 2016, 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.
+
+// test w/ `pub run test -N overridden_fields`
+
+class Base {
+  Object field = 'lorem';
+
+  Object something = 'change';
+}
+
+class Bad1 extends Base {
+  @override
+  final x = 1, field = 'ipsum'; // LINT
+}
+
+class Bad2 extends Base {
+  @override
+  Object something = 'done'; // LINT
+}
+
+class Bad3 extends Object with Base {
+  @override
+  Object something = 'done'; // LINT
+}
+
+class Ok extends Base {
+  Object newField; // OK
+
+  final Object newFinal = 'ignore'; // OK
+}
+
+class OK2 implements Base {
+  @override
+  Object something = 'done'; // OK
+
+  @override
+  Object field;
+}
+
+abstract class OK3 implements Base {
+  @override
+  Object something = 'done'; // OK
+}
+
+class GC11 extends Bad1 {
+  @override
+  Object something = 'done'; // LINT
+
+  Object gc33 = 'gc33';
+}
+
+abstract class GC12 implements Bad1 {
+  @override
+  Object something = 'done'; // OK
+}
+
+class GC13 extends Object with Bad1 {
+  @override
+  Object something = 'done'; // OK
+
+  @override
+  Object field = 'lint'; // LINT
+}
+
+abstract class GC21 extends GC11 {
+  @override
+  Object something = 'done'; // LINT
+}
+
+abstract class GC22 implements GC11 {
+  @override
+  Object something = 'done'; // OK
+}
+
+class GC23 extends Object with GC13 {
+  @override
+  Object something = 'done'; // LINT
+
+  @override
+  Object field = 'lint'; // LINT
+}
+
+class GC23_2 extends GC13 {
+  @override
+  var x = 7; // LINT
+}
+
+abstract class GC31 extends GC13 {
+  @override
+  Object something = 'done'; // LINT
+}
+
+abstract class GC32 implements GC13 {
+  @override
+  Object something = 'done'; // OK
+}
+
+class GC33 extends GC21 with GC13 {
+  @override
+  Object something = 'done'; // LINT
+
+  @override
+  Object gc33 = 'yada'; // LINT
+}
+
+class GC33_2 extends GC33 {
+  @override
+  var x = 3; // LINT
+
+  @override
+  Object gc33 = 'yada'; // LINT
+}
+
+class Super1 {}
+
+class Sub1 extends Super1 {
+  @override
+  int y;
+}
+
+class Super2 {
+  int x, y;
+}
+
+class Sub2 extends Super2 {
+  @override
+  int y; // LINT
+}
+
+class Super3 {
+  int x;
+}
+
+class Sub3 extends Super3 {
+  int x; // LINT
+}
+
+class A1 {
+  int f;
+}
+
+class B1 extends A1 {}
+
+abstract class C1 implements A1 {}
+
+class D1 extends B1 implements C1 {
+  @override
+  int f; // LINT
+}
+
+class A extends B {}
+class B extends A {
+  int field;
+}
+
+class StaticsNo {
+  static int a;
+}
+
+class VerifyStatic extends StaticsNo {
+  static int a;
+}
+
+mixin M on A1 {
+  @override
+  int f; // LINT
+
+  int g; // OK
+}
+
+abstract class BB {
+  abstract String s;
+}
+
+class AA extends BB {
+  /// Overriding abstracts in NNBD is OK.
+  @override
+  String s; // OK
+}
+
+class AAA with BB {
+  @override
+  String s; // OK
+}
+
+abstract class BBB {
+  abstract final String s;
+}
+
+class AAA extends BBB {
+  @override
+  String s; // OK
+}