fix parser error

frog/parser.dart

Index: frog/parser.dart
diff --git a/frog/parser.dart b/frog/parser.dart
index e130f30b2acd1141defb7f5647a098b3f4691cf0..4cc909646ada75f5c57b77cc046e5541e8832797 100644
--- a/frog/parser.dart
+++ b/frog/parser.dart
@@ -973,6 +973,10 @@ class Parser {
       var body = functionBody(true);
       return _makeFunction(expr, formals, body);
     } else {
+      if (expr is DeclaredIdentifier) {
+        _error('illegal target for call, did you mean to declare a function?',
+          expr.span);
+      }
       var args = arguments();
       return finishPostfixExpression(
           new CallExpression(expr, args, _makeSpan(expr.span.start)));