Fix test breakage in checked mode

tests/language/string_split_test.dart

 // Copyright (c) 2012, 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.
 // Issue 24043.
 
 import "package:expect/expect.dart";
 
 class EvilMatch implements Match {
   int get start => 100000000;
   int get end => 3;
 }
 
 class EvilIterator implements Iterator {
   bool moveNext() => true;
   EvilMatch get current => new EvilMatch();
 }
 
 class EvilIterable extends Iterable {
   Iterator get iterator => new EvilIterator();
 }
 
-class EvilPattern {
+class EvilPattern implements Pattern {
   Iterable allMatches(String s) => new EvilIterable();
 }
 
 void main() {
   Expect.throws(() => "foo".split(new EvilPattern())[0].length,
       (e) => e is RangeError);
 }