(This is related to #1757.)

The e2e tests are a bit flaky -- particularly the ones for editorAndPlayer.js -- and we've started to run into a pattern on Travis-CI where one of the 20 or so tests fails, the whole thing is restarted, and then a different test fails. This is quite irritating.

It might therefore be a good idea to split the protractor tests on Travis into several separate checks. That way, if one of them fails, only that set of checks needs to be re-run. The main caveat here is that we should have a mechanism to ensure that every single e2e test is accounted for, even if new ones are added later -- this could perhaps be done in the form of a backend test that scans the .travis.yml file and the core/tests/protractor/ directory, and makes sure that every file in the latter is included somewhere in the former.

/cc @makoscafee @souravbadami @BenHenning @wxyxinyu @bbriggs