checkValidity method
      
String?
checkValidity(
    
- MethodInvocation node,
- List arguments,
- String? outerName,
- FormalParameterList outerArgs, {
- bool nameAndArgsGenerated = false,
- bool examplesRequired = false,
override
    Verify that this looks like a correct Intl.message invocation.
Implementation
String? checkValidity(MethodInvocation node, List arguments,
    String? outerName, FormalParameterList outerArgs,
    {bool nameAndArgsGenerated: false, bool examplesRequired: false}) {
  if (arguments.first is! StringLiteral) {
    return "Intl.message messages must be string literals";
  }
  return super.checkValidity(node, arguments, outerName, outerArgs,
      nameAndArgsGenerated: nameAndArgsGenerated,
      examplesRequired: examplesRequired);
}