test(e2e): run on Dartium, dart2js and sauce browsers

Command line runs will test Dartium by default if no BROWSERS are
specified.  If BROWSERS are specified, they all have to be local
browsers or all sauce browsers.  If you're only specifying sauce
browsers (e.g. SL_Chrome, SL_Firefox, etc.), you may optionally also
specify DartiumWithWebPlatform and this will run two instances of
Protractor.  (This is a limitation of Protractor's config.)

On Travis, the job for a specific SDK version tests both
DartiumWithWebPlatform as well as SL_Chrome.  SL_Firefox is currently
disabled because it's failing.  Note that prior to this commit, the
BROWSERS variable was always ignored.