Implement CallTracker.thisFor().

src/core/CallTracker.js

@@ -49,6 +49,19 @@ getJasmineRequireObj().CallTracker = function(j$) {
       return call ? call.args : [];
     };
 
+    /**
+     * Get the "this" object that was passed to a specific invocation of this spy.
+     * @name Spy#calls#thisFor
+     * @since 3.7.1
+     * @function
+     * @param {Integer} index The 0-based invocation index.
+     * @return {Object?}
+     */
+    this.thisFor = function(index) {
+      var call = calls[index];
+      return call ? call.object : undefined;
+    };
+
     /**
      * Get the raw calls array for this spy.
      * @name Spy#calls#all