Closes #1141

Bump min SDK in all packages for safety. Restore use of for-loop
elements in a top level variable since they work in this SDK.

Towards #1141

Work around the bug in the SDK by switching back to `..addAll`. After we
publish this version that works we can bump the min SDK.

Prepare to publish.

- Bump to feature version change in `test`.
- Drop `-dev` from pubspecs.
- Pin to latest `test_core` from `test`.

Add support for writing any reporter output to a file in addition to the
reporter on `stdout`. The `json` reporter is the primary use case.
Adds a `--file-reporter` CLI argument, and `file_reporters` yaml config
option.

- Add a `MultiplexingReporter` to allow wiring up multiple reporters to
  listen at once.
- Plumb through the configuration mapping a reporter to a file path.
- Instantiate each file reporter with a sink from the file instead of
  the normal `stdout`.

Remove TODO comments waiting for things that will never change or that
we don't intend to take action on.

- https://github.com/dart-lang/sdk/issues/22977 is closed as not
  planned.
- Dart 2 does not support importing `dart:io` in cross platform code.
- Setting the exit code is not supported on web.
- We plan on keeping `executable.dart` under lib to allow reuse from
  other packages internally.
- We don't plan on trying to support `spawnHybridUri` outside of the
  test runner.
- `executable.main` is now declared to return a `Future`.
- We don't intend to take a dependency on `json_schema`.

Flutter pins package dependencies, including all transitive dependencies.

Because test_api depends on pedantic, Flutter pins that dependency.

However, pedantic is only depended on for the `unawaited` function, which is a one-line piece of code.

This will enable resolution of https://github.com/flutter/flutter/issues/48246 without requiring Flutter to unpin its dependencies, and with a very small overhead on test_api to not re-import pedantic :)  This helps downstream consumers who want to depend on the package for the actual analysis options files.

Also related: https://github.com/dart-lang/pedantic/issues/46

This allows for reusing one template file across all tests in use cases
where external scripts or html elements are required for all tests. The
possibility to still use local html files per test file is retained.

Fixes https://github.com/dart-lang/test/issues/39

This lint may soon be enforced in `package:pedantic`.

* Extend synthetic test timeout

* Add test-randomize-ordering-seed arg

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!

Fixes #1082 

Also fix readme

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 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.

pkg:vm_service_lib is discontinued

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

* Depend on latest analyzer

* remove unnecessary quotes

* widen range

Closes https://github.com/dart-lang/test/issues/1032

* Non-completed tests should cause failures

The default reporter is imported from `package:test/test.dart` and so it
is used on all platforms.

This change may mean that some unnecessary stack traces are printed,
which is safe and only slightly degrades UX.

Added `backend.dart` files to `test_core` and `test_api` that expose some new classes for use with the new `parseMetadata` function.

Version `1.7.0` adds the import so it's the minimum version required.
Allow versions through `2.x` since the breaking change will be to change
the import which will now be satisfied.

* prep for release

Fixes https://github.com/dart-lang/test/issues/943