[CFE] Remove inconsistency where visitIsExpression called .accept directly

Change-Id: I81d0e99e6908f24d082cfa57454db8fdc3b333be
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/166625
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

Author: jensjoha

pkg/front_end/lib/src/fasta/kernel/constant_evaluator.dart

@@ -2361,9 +2361,7 @@ class ConstantEvaluator extends RecursiveVisitor<Constant> {
 
   @override
   Constant visitIsExpression(IsExpression node) {
-    // TODO(jensj): Why does this call .accept directly?
-    // (@askesc says it's probably an oversight)
-    final Constant constant = node.operand.accept(this);
+    final Constant constant = _evaluateSubexpression(node.operand);
     if (constant is AbortConstant) return constant;
     if (shouldBeUnevaluated) {
       return unevaluated(