Support "key in map" using extension contains() function. #KT-3607 Fixed

libraries/stdlib/src/kotlin/collections/Maps.kt

@@ -20,6 +20,7 @@ public fun <K, V> MutableMap<K, V>.set(key : K, value : V) : V? = this.put(key,
 public fun <K,V> Map<K,V>?.orEmpty() : Map<K,V>
 = if (this != null) this else Collections.emptyMap<K,V>() as Map<K,V>
 
+public fun <K,V> Map<K,V>.contains(key : K) : Boolean = containsKey(key)
 
 /** Returns the key of the entry */
 public val <K,V> Map.Entry<K,V>.key : K

libraries/stdlib/test/collections/MapTest.kt

@@ -105,6 +105,12 @@ class MapTest {
         assertEquals("beverage,beer,location,Mells,name,James", list.makeString(","))
     }
 
+    test fun contains() {
+        val map = hashMapOf("a" to 1, "b" to 2)
+        assert("a" in map)
+        assert("c" !in map)
+    }
+
     test fun map() {
         val m1 = TreeMap<String, String>()
         m1["beverage"] = "beer"