add new unit test for used and unused imports

tests/cases/fourslash/referencesForImports.ts

@@ -0,0 +1,23 @@
+/// <reference path='fourslash.ts'/>
+
+////declare module "jquery" {
+////    function $(s: string): any;
+////    export = $;
+////}
+
+
+////import /*1*/$ = require("jquery");
+/////*2*/$("a");
+
+
+////import /*3*/$ = require("jquery");
+
+
+goTo.marker("1");
+verify.referencesCountIs(2);
+
+goTo.marker("2");
+verify.referencesCountIs(2);
+
+goTo.marker("3");
+verify.referencesCountIs(1);