Fix exception in linter (issue 315)

test/rules/parameter_assignments.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // test w/ `pub run test -N parameter_assignments`
 
 void badFunction(int parameter) { // LINT
   parameter = 4;
 }
 
 void ok(String parameter) {
   print(parameter);
 }
 
+String get topLevelGetter => '';
+
 class A {
   int get x => 0;
 
   set x(int value) { // LINT
     value = 5;
   }
 
   void badFunction(int parameter) { // LINT
     parameter = 4;
   }
@@ skipping 11 matching lines @@
 }
 
 void otherBad(int parameter) { // LINT
   print(parameter++);
 }
 
 void otherBad1(int parameter) { // LINT
   parameter += 3;
   print(parameter);
 }