Preserve space between "?" and "." in null aware element dot shorthand. (#1748)

Preserve space between "?" and "." in null aware element dot shorthand.

Fix #1642.

Author: munificent

lib/src/front_end/ast_node_visitor.dart

@@ -1587,7 +1587,18 @@ final class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
 
   @override
   void visitNullAwareElement(NullAwareElement node) {
-    writePrefix(node.question, node.value);
+    // A null-aware element containing a dot shorthand means there is a `?` and
+    // `.` next to each other. In that case, make sure we put a space between
+    // them so that they don't incorrectly get collapsed into a `?.` null-aware
+    // access token.
+    var space = switch (node.value) {
+      DotShorthandConstructorInvocation() => true,
+      DotShorthandInvocation() => true,
+      DotShorthandPropertyAccess() => true,
+      _ => false,
+    };
+
+    writePrefix(node.question, space: space, node.value);
   }
 
   @override

test/tall/expression/collection_null_aware.stmt

@@ -27,3 +27,17 @@ var list = [
   ?(veryLongExpression +
       thatIsForcedToSplit),
 ];
+>>> (experiment dot-shorthands) Preserves space for dot shorthand.
+### If the space between `?` and `.` is removed, it would become a single `?.`
+### token and thus a parse error.
+var list = [
+  ?  .  property  ,
+  ?  .  invocation  (  )  ,
+  ?  .  new  (  )  ,
+];
+<<< 3.10
+var list = [
+  ? .property,
+  ? .invocation(),
+  ? .new(),
+];

test/tall/expression/collection_null_aware_comment.stmt

@@ -33,3 +33,20 @@ var map = {
       ? // c
       value,
 };
+>>> (experiment dot-shorthands) Comment between `?` and `.` of dot shorthand.
+var list = [
+  ?  /* c */  .  property  ,
+  ?  // c
+  .  invocation  (  )  ,
+  ?  /// c
+  .  new  (  )  ,
+];
+<<< 3.10
+var list = [
+  ? /* c */ .property,
+  ? // c
+  .invocation(),
+  ?
+  /// c
+  .new(),
+];