Closes #824

Makes test/frontend/never_called_test work with --preview-dart-2

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

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

Closes #786

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

Closes #781

See the internal bug b/73723892

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.

We weren't destroying our references to `iframe`s after removing those
frames from the DOM, so we would slowly accumulate more and more
frames as time went on.

Partially addresses #724

Closes #753

Closes #742

Closes #740

Fix a couple of strong mode runtime cast errors.

In strong mode, a generic type instantiated with dynamic is not a
subtype of all types. You can't pass a List<dynamic> to something
expecting, say, List<int>.

These errors are usually detected statically, and most of those have
been fixed. However, sometimes this becomes a runtime cast, as in:

main() {
  // Store a List<dynamic> in a variable of type dynamic.
  dynamic d = [];

  // Implicit runtime downcast from dynamic to List<String>.
  List<String> s = d;
}

In order to ease the migration to strong mode, DDC has been ignoring
these cast failures when they involve certain commonly used types. We
are now in the process of actively fixing those errors.

More context: https://github.com/dart-lang/sdk/issues/27223

We were previously communicating over standard in/out, but this was
fragile: if raw JS code printed anything to stdout, it would break our
connection.

Closes #726

Closes #417

addTearDown() now adds a tearDownAll() rather than adding a plain
tearDown() when called from within setUpAll() (or tearDownAll()). This
is technically a behavioral change, but the old behavior was so
useless I doubt anyone was relying on it.

Closes #712

Use Stdout.terminalColumns for line length

Fall back on a 200-character default, which is less likely to obscure
test names when printing to a non-terminal destination.

Closes #86 

This makes it possible to use the same spawnHybridUri() call from
multiple test files that are in different places in the filesystem.

See #391