feat(LRUCache): Support zero-length caches

lib/core/cache.dart

@@ -82,7 +82,7 @@ class LruCache<K, V> extends Cache<K, V> {
   int _misses = 0;
 
   LruCache({int capacity}) {
-    this._capacity = (capacity == null) ? 0 : capacity;
+    this._capacity = capacity;
   }
 
   V get(K key) {
@@ -101,13 +101,12 @@ class LruCache<K, V> extends Cache<K, V> {
   V put(K key, V value) {
     // idempotent.  needed to refresh an existing key.
     _entries.remove(key);
-    // _capacity always > 0 but might not be true in some future.
-    if (_capacity > 0 && _capacity == _entries.length) {
+    _entries[key] = value;
+    if (_capacity != null && _capacity < _entries.length) {
       // drop oldest entry when at capacity
       // _entries.keys.first is fairly cheap - 2 new calls.
       _entries.remove(_entries.keys.first);
     }
-    _entries[key] = value;
     return value;
   }
 

lib/core_dom/module_internal.dart

@@ -48,7 +48,8 @@ class CoreDomModule extends Module {
     value(dom.Window, dom.window);
     value(ElementProbe, null);
 
-    factory(TemplateCache, (_) => new TemplateCache(capacity: 0));
+    // Default to a unlimited-sized TemplateCache
+    factory(TemplateCache, (_) => new TemplateCache());
     type(dom.NodeTreeSanitizer, implementedBy: NullTreeSanitizer);
 
     type(TextMustache);

test/core/cache_spec.dart

@@ -147,6 +147,12 @@ void main() {
         expect(stats.hits).toEqual(2);
         expect(stats.misses).toEqual(1);
       }));
+
+      it('should hold nothing if capacity is zero', () {
+        var cache = new LruCache<int, int>(capacity: 0);
+        cache.put(1, 10);
+        expect(cache.get(1)).toBeNull();
+      });
     });
   });
 }