fix(cache): Make UnboundedCache extend Cache

Closes #1174

Author: vicb

lib/cache/cache.dart

@@ -47,7 +47,7 @@ abstract class Cache<K, V> {
 /**
  * An unbounded cache.
  */
-class UnboundedCache<K, V> implements Cache<K, V> {
+class UnboundedCache<K, V> extends Cache<K, V> {
   Map<K, V> _entries = new HashMap<K, V>();
   int _hits = 0;
   int _misses = 0;

test/core_dom/http_spec.dart

@@ -8,9 +8,8 @@ var VALUE = 'val';
 var CACHED_VALUE = 'cached_value';
 
 class FakeCache extends UnboundedCache<String, HttpResponse> {
-  get(x) => x == 'f' ? new HttpResponse(200, CACHED_VALUE) : null;
-  put(_,__) => null;
-
+  HttpResponse get(x) => x == 'f' ? new HttpResponse(200, CACHED_VALUE) : null;
+  HttpResponse put(_,__) => null;
 }
 
 class SubstringRewriter extends UrlRewriter {