Issue 42385. Fix for analyzing 'prefix?.foo'.

Bug: https://github.com/dart-lang/sdk/issues/42385
Change-Id: I4dabfbda01a37210c57cb5ba6b5a7d530752e029
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151628
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -5395,12 +5395,10 @@ class ErrorVerifier extends RecursiveAstVisitor<void> {
       return false;
     }
 
-    // For `C?.foo()` the type of `C` is not set, it is not an expression.
+    // For `foo?.bar`, `foo` must be an identifier with a value.
     if (node is Identifier) {
       var element = node.staticElement;
-      if (element is ClassElement || element is ExtensionElement) {
-        return false;
-      }
+      return element is PropertyAccessorElement || element is VariableElement;
     }
 
     return true;

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

@@ -97,6 +97,24 @@ f(int? x) {
 ''');
   }
 
+  /// Here we test that analysis does not crash while checking whether to
+  /// report [StaticWarningCode.INVALID_NULL_AWARE_OPERATOR]. But we also
+  /// report another error.
+  test_getter_prefix() async {
+    newFile('/test/lib/a.dart', content: r'''
+int x = 0;
+''');
+    await assertErrorsInCode('''
+import 'a.dart' as p;
+
+f() {
+  p?.x;
+}
+''', [
+      error(CompileTimeErrorCode.PREFIX_IDENTIFIER_NOT_FOLLOWED_BY_DOT, 31, 1),
+    ]);
+  }
+
   test_index_legacy() async {
     newFile('/test/lib/a.dart', content: r'''
 // @dart = 2.5
@@ -287,4 +305,22 @@ f() {
 }
 ''');
   }
+
+  /// Here we test that analysis does not crash while checking whether to
+  /// report [StaticWarningCode.INVALID_NULL_AWARE_OPERATOR]. But we also
+  /// report another error.
+  test_setter_prefix() async {
+    newFile('/test/lib/a.dart', content: r'''
+int x = 0;
+''');
+    await assertErrorsInCode('''
+import 'a.dart' as p;
+
+f() {
+  p?.x = 0;
+}
+''', [
+      error(CompileTimeErrorCode.PREFIX_IDENTIFIER_NOT_FOLLOWED_BY_DOT, 31, 1),
+    ]);
+  }
 }