Fix typescript 1.6 compile errors.

- Remove unknown properties in object literals.
- Don't use module-level `delete` statements.

apps/tests/layouts/stack-layout-tests.ts

@@ -29,11 +29,11 @@ export function setUpModule() {
 
 export function tearDownModule() {
     navHelper.goBack();
-    delete tmp;
-    delete newPage;
-    delete rootLayout;
-    delete btn1;
-    delete btn2;
+    tmp = null;
+    newPage = null;
+    rootLayout = null;
+    btn1 = null;
+    btn2 = null;
 }
 
 export function setUp() {