Made members only used within a library private

There are no imports to this package.

- Delete dartium and content shell platforms
- Delete tests related to dartium
- Remove references to these platforms in docs
- Stop installing dartium on Travis

I'm not sure why the analyzer doesn't catch these, they appear to be
parse time bugs not runtime bugs when executing in the vm with
--preview-dart-2

Usage of 1.23 is non-existent
Increase turn-around on Travis

Fixes https://github.com/dart-lang/test/issues/800

Closes #786

Closes #784

Fix tests broken by dart2js using the CFE

I forgot that this wasn't supported in Dart 1.24 .

Closes #781

See the internal bug b/73723892

These tests have been flaking all over the place, and with the total
lack of support for content shell coming so soon it doesn't make sense
to spend the resources tracking down and fixing the root cause.

Add a SuitePlatform class

This clarifies the distinction between TestPlatform and SuitePlatform,
and uses a more specific word rather than a more general one.
Unfortunately, it does mean that we have some user-facing
interfaces (the --platform flag, the override_platforms and
define_platforms configuration fields) that now use different
terminology than the test runner.

It would be possible to migrate those to refer to "runtime" as well,
but seeing as --platform in particular is very widely-used *and*
matches up nicely with --preset (-p vs -P) I'm inclined to think it's
not worth the effort.

This encapsulates all the information about the platform on which a
suite runs, and which platform selectors can select for. Currently it
just includes the runtime and the operating system, but eventually it
could include whether we're running internally to Google, or which
compiler is in use.

Also re-enable headless mode by default.

See #772 for an example of a failure that only occurs in headless
mode.

This wasn't caught by Travis due to our workaround for #599.

Clean up the way source maps are passed to workers