Towards #1100

The ExpandedReporter needs to work on the web platform. It worked before
because it only used `print` which works everywhere.

- Change the argument to `StringSink` which can be supported on every
  platform.
- Remove the `dart:io` import from the expanded reporter.
- Make the `sink` argument non-optional for the expanded reporter.
- Add a `PrintSink` implementation that buffers writes until the content
  ends with a newline and then calls `print`.

Towards #1100 

Prepare for a world where we a reporter can be configured to write directly to a file.

* better solo

* Conditionally import coverage logic

Open to feedback on this. Here's how it works at the moment:

There's a new `--coverage` option that you can use to specify the output directory for your coverage. It will create a new output directory if one doesn't already exist.

The tests are run through the engine, and then get spit out to a coverage helper function. That coverage helper function:
**A)** Checks if it's a VM suite
**B)** Gathers coverage if it is
**C)** Outputs coverage to `${suite path}.vm.json`

So, for example, when I run:
```bash
pub run test --coverage hello_world test/vm/simple_repo_test.dart
```

A new file gets added at `hello_world/test/vm/simple_repo_test.dart.vm.json` with the coverage results!

Closes #1086 

Fixes #1082 

Also fix readme

* Fix chain tests

* default flags

* fix version

Mark internal packages as deprecated to hopefully prevent accidental use.

* Update test to use analyzer 0.38.0

* Update test_core dep

* Add a dependency override for test_core

This was added in https://github.com/dart-lang/test/pull/729 when the
`Socket` class was missing a method in an interface it declared it had -
this caused a problem when some code tried to tear if off.

According to https://github.com/dart-lang/sdk/issues/31504 that method
now exists and can be torn off - calling that method would still throw,
but that would also be true with a `DelegatignStreamSink` wrapper.

This file is unused since there is another copy in `test_api` which is
used instead.

This had been used to detect feature support so that the package could
have a wider SDK constraint and have enhanced features for newer SDKs.
Drop the dependency and code checking the SDK version because:

- We now typically bump the SDK constraint rather than support multiple
  code paths.
- The remaining usage was for checking for DDC support in `pub serve`
  which doesn't exist anymore - we haven't yet deleted the tests or
  migrated them to use `build_runner serve`. This test and references to
  the old approach with both compilers is retained to make it clear what
  should be tested when we get around to migrating it.

The old flag, `--categories=Server`, is deprecated and emits a warning
when used. The new flag is supported since at least 2.1.1

Since our pubspec already limits us to at least 2.2.0, remove the SDK
version check for using the flag.

pkg:vm_service_lib is discontinued

Related to https://github.com/dart-lang/vm_service_drivers/issues/257

- Remove `removeAllOutstandingCallbacks` method. This never had any
  usage outside of `scheduled_test` which has been deprecated for a long
  time.
- Change some bare `Future` to `Future<void>`.
- Move and rename `OutstandingCallbackCounter` class - the
  implementation has no ties to the concept of callbacks, but that is
  how it is used.
- Use the modern syntax for function typed arguments.
- Add generics to `unclosable` to void throwing away potentially useful
  type information.

* Depend on latest analyzer

* remove unnecessary quotes

* widen range

A recent change to the Dart SDK updated `HttpClientResponse`
to implement `Stream<Uint8List>` rather than implementing
`Stream<List<int>>`.

This forwards-compatible chnage updates calls to
`Stream.transform(StreamTransformer)` to instead call the
functionally equivalent `StreamTransformer.bind(Stream)`
API, which puts the stream in a covariant position and
thus causes the SDK change to be non-breaking.

https://github.com/dart-lang/sdk/issues/36900

Since analyzer version 0.36.0, we no longer need to use elements2.
elements is equivalent.