fixed `iron-image-test`.
fixed `iron-image-test`.

splitted `--iron-image-width` and `--iron-image-height` tests
because `fixture` will allow only one fixture per suite.

dart-archive/polymer_elements,dart-archive/polymer_elements,dart-archive/polymer_elements,dart-archive/polymer_elements,dart-archive/polymer_elements,dart-archive/polymer_elements