Template strings not transpiled correctly in some cases

When there is

1. An interpolation expression that is not a single symbol
2. followed immediately by another interpolation:

const str = `${a + b}${c}`

The first expression should be wrapped by a pair of parens, but the closing paren is missing:

https://buble.surge.sh/#var%20a%20%3D%20%60%24%7Ba%20%2B%20b%7D%24%7Bc%7D%60