slightly improve Map.keys/Map.items (see #6063)

language/src/ceylon/language/Map.ceylon

@@ -119,20 +119,24 @@ shared interface Map<out Key=Object, out Item=Anything>
     shared actual formal Map<Key,Item> clone();
 
     "A [[Collection]] containing the keys of this map."
+    //TODO: should be a Set
     shared actual default Collection<Key> keys
-            => object satisfies Collection<Key> {
+            => object extends Object() 
+                      satisfies Set<Key> {
         contains(Object key) => outer.defines(key);
         iterator() => outer.map(Entry.key).iterator();
-        clone() => [*this];
         size => outer.size;
+        empty => outer.empty;
+        clone() => set(this);
     };
 
     "A [[Collection]] containing the items stored in this 
      map. An element can be stored under more than one key 
      in the map, and so it can occur more than once in the 
      resulting collection."
     shared default Collection<Item> items
-            => object satisfies Collection<Item> {
+            => object extends Object() 
+                      satisfies Collection<Item> {
         shared actual Boolean contains(Object item) {
             for (k->v in outer) {
                 if (exists v, v==item) {
@@ -144,8 +148,9 @@ shared interface Map<out Key=Object, out Item=Anything>
             }
         }
         iterator() => outer.map(Entry.item).iterator();
-        clone() => [*this];
         size => outer.size;
+        empty => outer.empty;
+        clone() => [*this];
     };
 
     "Invert this map, producing a new immutable map where