fixUnusedIdentifier may remove a used side-effect

[ghost]

TypeScript Version: 2.9.0-dev.20180506

Code

{ const x = launchMissiles(); }

Expected behavior:

const x = removed, and missiles still launched.

Actual behavior:

Missiles unlaunched.

Unfortunately we have no way of knowing whether a function call does have side effects (#17181), so doing this in general might be annoying as users have to manually delete function calls with unused results.