Add missing ArrayBuilder.cs file

src/Common/src/System/Collections/Generic/ArrayBuilder.cs

@@ -0,0 +1,48 @@
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+
+namespace System.Collections.Generic
+{
+    //
+    // Helper class for building lists that avoids unnecessary allocation
+    //
+    internal struct ArrayBuilder<T>
+    {
+        T[] _items;
+        int _count;
+
+        public T[] ToArray()
+        {
+            if (_items == null)
+                return Array.Empty<T>();
+            if (_count != _items.Length)
+                Array.Resize(ref _items, _count);
+            return _items;
+        }
+
+        public void Add(T item)
+        {
+            if (_items == null || _count == _items.Length)
+                Array.Resize(ref _items, 2 * _count + 1);
+            _items[_count++] = item;
+        }
+
+        public int Count
+        {
+            get
+            {
+                return _count;
+            }
+        }
+
+        public T this[int index]
+        {
+            get
+            {
+                return _items[index];
+            }
+        }
+    }
+}