Define plusAssign operator for mutable collections.

#KT-4020 Fixed

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

@@ -1,5 +1,27 @@
 package kotlin
 
+/**
+ * Adds the specified [element] to this mutable collection.
+ */
+public fun <T> MutableCollection<in T>.plusAssign(element: T) {
+    this.add(element)
+}
+
+/**
+ * Adds all elements of the given [collection] to this mutable collection.
+ */
+public fun <T> MutableCollection<in T>.plusAssign(collection: Iterable<T>) {
+    this.addAll(collection)
+}
+
+/**
+ * Adds all elements of the given [array] to this mutable collection.
+ */
+public fun <T> MutableCollection<in T>.plusAssign(array: Array<T>) {
+    this.addAll(array)
+}
+
+
 /**
  * Adds all elements of the given [iterable] to this [MutableCollection].
  */

libraries/stdlib/test/collections/CollectionTest.kt

@@ -233,6 +233,12 @@ class CollectionTest {
         l += arrayOf("cheese", "wine")
         assertEquals(listOf("cheese", "foo", "beer", "cheese", "wine"), l)
         assertTrue(l !== lOriginal)
+
+        val ml = arrayListOf("cheese")
+        ml += "foo"
+        ml += listOf("beer")
+        ml += arrayOf("cheese", "wine")
+        assertEquals(l, ml)
     }
 
     test fun requireNoNulls() {