Report original test location in the json_reporter if different (dart…

…-lang#853)

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

* update docs, pubspec, changelog

More Dart 2 runtime semantic fixes (dart-lang#844)

```
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.

Remove transformer and barback dependency (dart-lang#823)

Expose bootstrap methods publicly (dart-lang#820)

Pass --categories=Server to dart2js for Node

Workaround for async SDK issue (dart-lang#787)

Stop importing dart:io from browser libraries

Add the ability to include one config file from another