You could use the new hasProperty matcher in mirror_matchers.dart, and say:

  expect(size, hasProperty('height', 400));

This can give you better error messages.

Or you could use 'reason':

   expect(size.height, 400, reason: 'window height should be 400');

Not a big deal but otherwise the messages you get from failures here will not be as clear (something like 'Expected <400>: Actual: <300>');