Added tests for index signature completion builders.

Author: DanielRosenwasser

tests/cases/fourslash/completionListInIndexSignature01.ts

@@ -0,0 +1,20 @@
+/// <reference path='fourslash.ts' />
+
+////interface I<T> {
+////    [/*1*/]: T;
+////    [/*2*/]: T;
+////}
+////
+////class C {
+////    [/*3*/]: string;
+////    [str/*4*/: string]: number;
+////}
+////
+////type T = {
+////    [x/*5*/yz: number]: boolean;
+////    [/*6*/
+
+for (let marker of test.markers()) {
+    goTo.position(marker.position);
+    verify.completionListAllowsNewIdentifier();
+}

tests/cases/fourslash/completionListInIndexSignature02.ts

@@ -0,0 +1,19 @@
+/// <reference path='fourslash.ts' />
+
+////interface I<T> {
+////    [x: /*1*/]: T;
+////    [: /*2*/]: T
+////}
+////
+////class C {
+////    [a: /*3*/]: string;
+////    [str: string/*4*/]: number;
+////}
+////
+////type T = {
+////    [xyz: /*5*/
+
+for (let marker of test.markers()) {
+    goTo.position(marker.position);
+    verify.not.completionListAllowsNewIdentifier();
+}