Fix incorrect nullability inference for ??

Fix bug where (a?.foo) was being incorrectly inferred to be
non-nullable if `foo` was non-nullable.

BUG=
R=jmesserly@google.com

Review-Url: https://codereview.chromium.org/2996523003 .

pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart

@@ -59,7 +59,8 @@ abstract class NullableTypeInference {
     // resulting value if that becomes an issue, so we maintain the invariant
     // that each node is visited once.
     Element element = null;
-    if (expr is PropertyAccess) {
+    if (expr is PropertyAccess &&
+        expr.operator?.type != TokenType.QUESTION_PERIOD) {
       element = expr.propertyName.staticElement;
     } else if (expr is Identifier) {
       element = expr.staticElement;

tests/language_strong/nullaware_opt_test.dart

@@ -79,10 +79,20 @@ class Foo {
   Foo(this._bar) : str = _bar?.s;
 }
 
+// Check that ?? isn't incorrectly optimized as non-nullable
+// (DDC regression test)
+test3() {
+  List n = null;
+  var func = n?.add;
+  var result = func ?? 1;
+  Expect.equals(result, 1);
+}
+
 main() {
   for (int i = 0; i < 10; i++) {
     test();
     test2();
+    test3();
   }
 
   Expect.equals(null, new Foo(new Bar()).str);