Array indexing with strictNullChecks should return ItemType | undefined

[mmkal]

TypeScript Version: 2.2.2

Code

const words = new Array<string>();
words.push("foo");
const bar = words[2];

Expected behavior:
When strictNullChecks is turned on, the type of bar should be string | undefined. In the case above, it will definitely be undefined.

Actual behavior:
The type is string.

[ahejlsberg]

Duplicate of #13778.