fix typo in dart:core.Map docs

sdk/lib/core/map.dart

Index: sdk/lib/core/map.dart
diff --git a/sdk/lib/core/map.dart b/sdk/lib/core/map.dart
index e92c780aedfc8351b92dfd70dc759499ba7ca8b6..4f7fcf8220816a8d6e04e6935d2c2450493f91df 100644
--- a/sdk/lib/core/map.dart
+++ b/sdk/lib/core/map.dart
@@ -149,7 +149,7 @@ abstract class Map<K, V> {
   /**
    * Returns true if this map contains the given [key].
    *
-   * Returns true if any of the keys in the map ar equal to `key`
+   * Returns true if any of the keys in the map are equal to `key`
    * according to the equality used by the map.
    */
   bool containsKey(Object key);