Add generic version of Array.isArray

src/typings/lib.array-ext.d.ts

@@ -0,0 +1,8 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+interface ArrayConstructor {
+    isArray<T>(arg: any): arg is Array<T>;
+}