Mocha test suite halted on uncaught exception

I can't prevent an uncaught async exception from halting a suite of mocha tests. It seems like it should fail just the test, but the mocha process exits. For the given test cases:

describe('AsyncWow', () => {
    it('should proceeed after this test', (done) => {
        setTimeout(() => {
            throw new Error('boom');
            done();
        });
    });
    it('but never gets here', (done) => {
        done();
    });
});

Produces terminal output:

  AsyncWow

/source/test/foo.test.js:6
         throw new Error('boom');
               ^
Error: boom
    at [object Object]._onTimeout (/source/test/foo.test.js:6:16)
    at Timer.listOnTimeout (timers.js:110:15)