fixed issue #18.

Author: Dummy

dist/spec/issues/15.test.js

@@ -0,0 +1 @@
+export {};

dist/spec/issues/15.test.js.map

@@ -1,6 +1,7 @@
 export declare class Argument<T> {
     private description;
     private matchingFunction;
+    encounteredValues: any[];
     constructor(description: string, matchingFunction: (arg: T) => boolean);
     matches(arg: T): boolean;
     toString(): string;