Issue 40704. Support for ?[ in resolution.

Bug: dart-lang/sdk#40704
Change-Id: Iaa4680e65a44c83bdc6a3ae826cc510b685a7ccf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138563
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -5853,6 +5853,7 @@ class IndexExpressionImpl extends ExpressionImpl
 
   @override
   bool get isNullAware =>
+      question != null ||
       leftBracket.type == TokenType.QUESTION_PERIOD_OPEN_SQUARE_BRACKET ||
       (leftBracket.type == TokenType.OPEN_SQUARE_BRACKET &&
           period != null &&

pkg/analyzer/lib/src/dart/ast/to_source_visitor.dart

@@ -741,13 +741,14 @@ class ToSourceVisitor implements AstVisitor<void> {
   @override
   void visitIndexExpression(IndexExpression node) {
     if (node.isCascaded) {
-      sink.write(node.period.lexeme);
+      safelyVisitToken(node.period);
     } else {
       safelyVisitNode(node.target);
     }
-    sink.write('[');
+    safelyVisitToken(node.question);
+    safelyVisitToken(node.leftBracket);
     safelyVisitNode(node.index);
-    sink.write(']');
+    safelyVisitToken(node.rightBracket);
   }
 
   @override