fixUnusedIdentifier may remove a used side-effect

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.

Start by locating the fixUnusedIdentifier entry point and reproduce the issue with the provided { const x = launchMissiles(); } example. Determine how unused identifiers are removed while preserving the function call's possible side effect, and verify that the compiled output still launches the missiles.