- Fix failing `services/api.js` test

  Chrome throws an error when trying to construct a fetch `Response` with
  a non-null `body` if the status is 204.

- Fix Timestamp and GroupListItem test failures in Chrome

  - Pass required type argument to `Event` constructor
  - Pass valid `timestamp` argument to `Timestamp`. This value is passed
    on to date functions which require a valid date/time string.

- Fix Slider transition test in Chrome

  Since the transition from open to closed is animated, the actual element
  height doesn't immediately transition to 0px. Instead of checking the
  actual height, check the target height specified by the current style
  properties.

  The previous code happened to work under PhantomJS, but was conceptually incorrect.