Allow arity checks on dynamic function types

lib/src/checker/checker.dart

Index: lib/src/checker/checker.dart
diff --git a/lib/src/checker/checker.dart b/lib/src/checker/checker.dart
index de0f0130519aee213dc809dc51a7bd8b872ac425..e88572ccb722d75115d2892c101fb0c9b4f50eb4 100644
--- a/lib/src/checker/checker.dart
+++ b/lib/src/checker/checker.dart
@@ -675,6 +675,9 @@ class CodeChecker extends RecursiveAstVisitor {
   }
 
   visitAsExpression(AsExpression node) {
+    // We could do the same check as the IsExpression below, but that is
+    // potentially too conservative.  Instead, at runtime, we must fail hard
+    // if the Dart as and the DDC as would return different values.
     node.visitChildren(this);
   }