Fix some cases where Map.containsValue did not accept Object as argument.

sdk/lib/collection/maps.dart

Index: sdk/lib/collection/maps.dart
diff --git a/sdk/lib/collection/maps.dart b/sdk/lib/collection/maps.dart
index 7945e7ea643411074c5e65769a5870c0733913c7..2f803883120f8e768b8d9757234104c3ba16e7db 100644
--- a/sdk/lib/collection/maps.dart
+++ b/sdk/lib/collection/maps.dart
@@ -60,7 +60,7 @@ abstract class MapMixin<K, V> implements Map<K, V> {
     }
   }
 
-  bool containsValue(V value) {
+  bool containsValue(Object value) {
     for (K key in keys) {
       if (this[key] == value) return true;
     }
@@ -218,18 +218,18 @@ class UnmodifiableMapView<K, V> =
  * necessary to implement each particular operation.
  */
 class Maps {
-  static bool containsValue(Map map, value) {
+  static bool containsValue(Map map, Object value) {
     for (final v in map.values) {
-      if (value == v) {
+      if (v == value) {
         return true;
       }
     }
     return false;
   }
 
-  static bool containsKey(Map map, key) {
+  static bool containsKey(Map map, Object key) {
     for (final k in map.keys) {
-      if (key == k) {
+      if (k == key) {
         return true;
       }
     }