Added null-check selector to syntax of cascades, and test.

Change-Id: Iecfe33abaf440576127f9a2b536593328c906e12
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124332
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Erik Ernst <eernst@google.com>

Author: eernstg

tests/language_2/nnbd/syntax/cascade_nullcheck_test.dart

@@ -0,0 +1,26 @@
+// Copyright (c) 2019, 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.
+
+// SharedOptions=--enable-experiment=non-nullable
+
+import "package:expect/expect.dart";
+
+class C {
+  int? y;
+  D? f() => new D();
+  C? h() => this;
+}
+
+class D {
+  D g() => this;
+  bool? operator [](String s) => true;
+}
+
+void test(C x) {
+  x..f()!.g()['Hi!']!..h()!.y = 2;
+}
+
+main() {
+  test(new C());
+}