Updated test to make new output of Type mismatch errors
Update usage of isInstanceOf to TypeMatcher

* add root_line, root_column, and root_package_url to the json reporter

* update docs, pubspec, changelog

* enable the compact reporter and colors on windows, if stdout.supportsAnsiEscapes == true

* update pubspec/changelog, code review updates

```
type 'WebSocketChannel' is not a subtype of type 'StreamChannel<String>' of 'channel'
```

Cast the `WebSocketChannel` to indicate that all values in each
direction will be `String`.

```
type 'CloseGuaranteeChannel<dynamic>' is not a subtype of type 'VirtualChannel'
```

Avoid using the `suiteChannel` variable for two types. It needs to start
as a `VirtualChannel` which has the `id` field, and then later be a
`StreamChannel`.

```
type 'RunnerSuite' is not a subtype of type 'FutureOr<Future<RunnerSuite>>'
```

Avoid the attempted Future flattening which was never correct but was
supported in the Dart 1 VM.

Prepare to release 0.12.38+1

- added `// ignore` comments for references to internal deprecated members
- removed comments referencing deprecated arguments
- upped args minimum constraint and migrated to `addMultiOption`
- dropped polymer support
- added `--fatal-infos` and `--fatal-warnings` to dartanalyzer test to prevent regressions and expose new warnings via cron jobs.
- fixed up some markdown issues

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