Adding fourslash test

Author: ahejlsberg

tests/cases/fourslash/quickInfoOnNarrowedType.ts

@@ -1,5 +1,7 @@
 /// <reference path='fourslash.ts'/>
 
+// @strictNullChecks: true
+
 ////function foo(strOrNum: string | number) {
 ////    if (typeof /*1*/strOrNum === "number") {
 ////        return /*2*/strOrNum; 
@@ -9,6 +11,13 @@
 ////    }
 ////}
 
+////function bar() {
+////   let s: string | undefined;
+////   /*4*/s;
+////   /*5*/s = "abc";
+////   /*6*/s;
+////}
+
 goTo.marker('1');
 verify.quickInfoIs('(parameter) strOrNum: string | number');
 verify.completionListContains("strOrNum", "(parameter) strOrNum: string | number");
@@ -20,3 +29,15 @@ verify.completionListContains("strOrNum", "(parameter) strOrNum: number");
 goTo.marker('3');
 verify.quickInfoIs('(parameter) strOrNum: string');
 verify.completionListContains("strOrNum", "(parameter) strOrNum: string");
+
+goTo.marker('4');
+verify.quickInfoIs('let s: undefined');
+verify.completionListContains("s", "let s: undefined");
+
+goTo.marker('5');
+verify.quickInfoIs('let s: string | undefined');
+verify.completionListContains("s", "let s: string | undefined");
+
+goTo.marker('6');
+verify.quickInfoIs('let s: string');
+verify.completionListContains("s", "let s: string");