Enhance Object.assign static method signature

Author: falsandtru

lib/lib.es2015.core.d.ts

@@ -298,7 +298,7 @@ interface ObjectConstructor {
       * @param target The target object to copy to.
       * @param source The source object from which to copy properties.
       */
-    assign<T, U>(target: T, source: U): T & U;
+    assign<T, U>(target: T, source: U, ...sources: U[]): T & U;
 
     /**
       * Copy the values of all of the enumerable own properties from one or more source objects to a