Expect a '(' before calling parseArguments (which asserts this is the case).

dart/frog/leg/scanner/parser.dart

Index: dart/frog/leg/scanner/parser.dart
diff --git a/dart/frog/leg/scanner/parser.dart b/dart/frog/leg/scanner/parser.dart
index e8d71a7a14c8d62111696cc8e3267283caca6ebf..81b7ec82331a31cbb83260adea3a70afe79b2cd5 100644
--- a/dart/frog/leg/scanner/parser.dart
+++ b/dart/frog/leg/scanner/parser.dart
@@ -1225,6 +1225,7 @@ class Parser {
       named = true;
       token = parseIdentifier(token.next);
     }
+    expect('(', token);
     token = parseArguments(token);
     listener.handleConstExpression(constKeyword, named);
     return token;