[analyzer] Dot shorthands: Fix missed isDotShorthand(node) usage.

Missed one refactor in the binary_expression_resolver.

Following up to comments on:
https://dart-review.googlesource.com/c/sdk/+/422941/12/pkg/analyzer/lib/src/dart/resolver/binary_expression_resolver.dart#111

Bug: #59835
Change-Id: Ia210ddc5141222e13bda163681e1ae2b05dc55bb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/423960
Commit-Queue: Kallen Tu <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: kallentu

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

@@ -108,7 +108,7 @@ class BinaryExpressionResolver {
 
     // When evaluating exactly a dot shorthand in the RHS, we save the LHS type
     // to provide the context type for the shorthand.
-    if (node.rightOperand is DotShorthandMixin) {
+    if (_resolver.isDotShorthand(node.rightOperand)) {
       _resolver.pushDotShorthandContext(
         node.rightOperand,
         SharedTypeSchemaView(left.typeOrThrow),