Fix exception in linter (issue 315)

test/rules/parameter_assignments.dart

Index: test/rules/parameter_assignments.dart
diff --git a/test/rules/parameter_assignments.dart b/test/rules/parameter_assignments.dart
index 4a95dfbd741c1a001fd5b0506e2fc0096838ae8f..84bd0f4f157f9748453ae0b041870e34372f373a 100644
--- a/test/rules/parameter_assignments.dart
+++ b/test/rules/parameter_assignments.dart
@@ -12,6 +12,8 @@ void ok(String parameter) {
   print(parameter);
 }
 
+String get topLevelGetter => '';
+
 class A {
   int get x => 0;