Don't report HintCode.CAN_BE_NULL_AFTER_NULL_AWARE when NNBD.

With NNBD we do shorting, so `foo?.bar.baz` is OK, `baz` will never be
requested from `null`, unless `foo.bar` is nullable, in which case
it is UNCHECKED_USE_OF_NULLABLE_VALUE.

R=brianwilkerson@google.com

Change-Id: Ic9bbd0f6116dfca53b8dd9cf05b557919c025d2d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/115762
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/generated/resolver.dart

@@ -93,6 +93,9 @@ class BestPracticesVerifier extends RecursiveAstVisitor<void> {
   /// The [LinterContext] used for possible const calculations.
   LinterContext _linterContext;
 
+  /// Is `true` if NNBD is enabled for the library being analyzed.
+  final bool _isNonNullable;
+
   /// Create a new instance of the [BestPracticesVerifier].
   ///
   /// @param errorReporter the error reporter
@@ -110,6 +113,7 @@ class BestPracticesVerifier extends RecursiveAstVisitor<void> {
   })  : _nullType = typeProvider.nullType,
         _futureNullType = typeProvider.futureNullType,
         _typeSystem = typeSystem ?? new Dart2TypeSystem(typeProvider),
+        _isNonNullable = unit.featureSet.isEnabled(Feature.non_nullable),
         _inheritanceManager = inheritanceManager,
         _invalidAccessVerifier =
             new _InvalidAccessVerifier(_errorReporter, _currentLibrary) {
@@ -918,6 +922,10 @@ class BestPracticesVerifier extends RecursiveAstVisitor<void> {
 
   /// Produce several null-aware related hints.
   void _checkForNullAwareHints(Expression node, Token operator) {
+    if (_isNonNullable) {
+      return;
+    }
+
     if (operator == null || operator.type != TokenType.QUESTION_PERIOD) {
       return;
     }

pkg/analyzer/test/src/diagnostics/unchecked_use_of_nullable_value_test.dart

@@ -805,8 +805,6 @@ m(B? b) {
   b.a.x; // 2
 }
 ''', [
-      // TODO(scheglov) Remove HintCode.CAN_BE_NULL_AFTER_NULL_AWARE
-      error(HintCode.CAN_BE_NULL_AFTER_NULL_AWARE, 86, 4),
       error(StaticWarningCode.UNCHECKED_USE_OF_NULLABLE_VALUE, 101, 1),
     ]);
     var propertyAccess1 = findNode.propertyAccess('x; // 1');
@@ -879,8 +877,6 @@ m(C c) {
   c.b.a.x; // 2
 }
 ''', [
-      // TODO(scheglov) Remove HintCode.CAN_BE_NULL_AFTER_NULL_AWARE
-      error(HintCode.CAN_BE_NULL_AFTER_NULL_AWARE, 131, 6),
       error(StaticWarningCode.UNCHECKED_USE_OF_NULLABLE_VALUE, 148, 3),
     ]);
     var propertyAccess1 = findNode.propertyAccess('x; // 1');