importHelpers does not emit import unless at least one other import is present

[Jessidhia]

Noticed on a webpack project with typescript@2.1.1, using either commonjs or es2015 modules:

// index.ts

import './empty'

async function x () {}

// empty.ts

This emits an import for tslib following by the import for empty. However, commenting the import 'empty' line out also omits the import for tslib, and the emitted code attempts to use a global __awaiter.

I thought this might have been fixed by #12151, but it's still present in current master (built with gulp LKG).

[Jessidhia]

Using isolatedModules gives a clue: the "Cannot compile namespaces" error hints that tsc doesn't/can't treat files without import/export as a module.

With isolatedModules enabled, putting an export {} is enough to pass the isolatedModules condition, and the code is emitted with the tslib import.

[Checksum]

Just wasted two hours on this! With an empty import statement, it works as intended. Tested on version 2.2.0-dev.20170110

[mhegazy]

If the file does not have a toplevel import/export is it is not considered a module, and it is treated as script/global.

use --isolatedModules to flag these files as errors.