Add AbstractSet from main Kotlin repo

runtime/src/main/kotlin/kotlin/collections/AbstractSet.kt

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2010-2016 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kotlin.collections
+
+/**
+ * Provides a skeletal implementation of the read-only [Set] interface.
+ *
+ * This class is intended to help implementing read-only sets so it doesn't support concurrent modification tracking.
+ *
+ * @param E the type of elements contained in the set. The set is covariant on its element type.
+ */
+@SinceKotlin("1.1")
+public abstract class AbstractSet<out E> protected constructor() : AbstractCollection<E>(), Set<E> {
+
+    /**
+     * Compares this set with other set instance with the unordered structural equality.
+     *
+     * @return true, if [other] instance is a [Set] of the same size, all elements of which are contained in this set.
+     */
+    override fun equals(other: Any?): Boolean {
+        if (other === this) return true
+        if (other !is Set<*>) return false
+        return setEquals(this, other)
+    }
+
+    /**
+     * Returns the hash code value for this set.
+     */
+    override fun hashCode(): Int = unorderedHashCode(this)
+
+    internal companion object {
+        internal fun unorderedHashCode(c: Collection<*>): Int {
+            var hashCode = 0
+            for (element in c) {
+                hashCode += (element?.hashCode() ?: 0)
+            }
+            return hashCode
+        }
+
+        internal fun setEquals(c: Set<*>, other: Set<*>): Boolean {
+            if (c.size != other.size) return false
+            return c.containsAll(other)
+        }
+    }
+
+}