improve assign() util performance

src/shared/utils.js

@@ -1,10 +1,11 @@
 export function noop () {}
 
 export function assign ( target ) {
-	for ( var i = 1; i < arguments.length; i += 1 ) {
-		var source = arguments[i];
-		for ( var k in source ) target[k] = source[k];
+	var k, source, i = 1, len = arguments.length;
+	for ( ; i < len; i++ ) {
+		source = arguments[i];
+		for ( k in source ) target[k] = source[k];
 	}
 
 	return target;
-}
+}