Incorrect elision of import {} with TS transform disabled

[alangpierce]

As I was working on import elision behavior, I noticed a bug when handling this code snippet:

import {} from "./a";

The expected behavior is:

• With the TS transform enabled, the import is elided, so ./a is never evaluated at runtime. TS playground link. This happens because "all imports are type-only" is vacuously true.
• With the TS transform disabled, the import is preserved, so ./a is evaluated at runtime. This is because import elision is a TS feature, so in plain JS there's no reason to do it.

Sucrase handles the first case correctly, but when the TS transform is disabled, it incorrectly removes the import, so this issue tracks the fix for that.

[alangpierce]

Another case to consider is the Flow behavior for this code. Babel and flow-remove-types have different import elision behavior in general (Babel removes an import if all named imports are removed, which flow-remove-types doesn't do), but they both preserve the import in this case, so I'll change Sucrase to do that. For flow cases like import {type a} from './a';, I'll keep Babel's behavior of removing the whole line, which is what Sucrase is already doing.