Run analyzer (at a minimum) across all packages at their oldest supported SDK

In one specific test we expect an executable to be able to run the tests
multiple times. For that case we don't want to hard exit ever.

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

Messages sent from iframes to the host via `window.postMessage()` are
tagged with the iframe's href so that the host can associate the message
with the sending iframe. This is necessary because messages sent from
all iframes are received by the same event handler.

However, this isn't necessary for messages sent over a `MessageChannel`,
as the host uses a separate `MessageChannel` for each iframe.

This prevents leaking subscriptions if the test is ever hot restarted.
This is especially important because the transferred message port's stream
is single subscription, and will throw if multiple active subscriptions
attempt to subscribe.

The host code that runs in the browser and loads tests now uses the
Channel Messaging API to communicate with iframes. It now connects to
each test suite's iframe with its own dedicated `MessageChannel`.

This method is more robust than sending global messages through
`window.postMessage()`.

It also allows tests to modify `window.location`, which would previously
cause them to hang. This was because each iframe used `window.location`
(which is unique to each iframe) to tag messages sent using
`window.postMessage()` so that the host could identify their source.

Closes https://github.com/dart-lang/test/issues/423.
Fixes https://github.com/dart-lang/test/issues/562.

- Don't surround versions with quotes unless they have a space.
- Remove the unnecessary lower bound for packages where the current
  lower bound is not resolvable due to SDK constraints.
- Change single quotes to double quotes since they are preferred in
  yaml.
- Remove version constraints from `test_api` to `test` and `test_core`
  since we will always have path overrides to those packages.

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.

As part of implementing the "UI as code" feature
(https://github.com/dart-lang/language/blob/master/accepted/future-releases/unified-collections/feature-specification.md)
the analyzer is deprecating the following classes and methods:

- ForStatement and ForEachStatement (ForStatement2 is now used for
  both kinds of loops)

- ListLiteral.elements (use elements2, which has a more general return
  type)

- AstVisitor.visitForEachStatement and AstVisitor.visitForStatement
  (visitForStatement2 is now used for both kinds of loops)

- AstVisitor.visitMapLiteral and AstVisitor.visitSetLiteral
  (visitSetOrMapLiteral is now used for both kinds of literals).

See deprecation CL: https://dart-review.googlesource.com/c/sdk/+/95665/

Gives an actionable error in a case where we'd otherwise get a stack
trace.

We have removed most of the special handling around
`IsolateSpawnException` so ti shouldn't matter if the exposed exception
implements that interface. On the web, for the `spawnHybridUri` or
`spawnHybridCode` case the web code shouldn't be able to see the type to
test for it anyway.

After splitting the package the file was copied to some of the package
but missing from the `test_core` package. Move it back up the directory
structure so it is picked up for all sub-packages.

Fix lints that were not caught in `test_core` while this was missing.

The linked SDK issue is resolved.

The SDK no longer outputs anything that looks like "Load Error". The
last commit which removed this pattern was
250773cb651beec1a61e126a6c0756aace9c19ca

I'm also unable to reproduce any case where the `toString()` includes
"Stack Trace:\n", and the file names don't look like they need to be
adjusted at all.

I will separetly look in to the migration, this is to get travis
unblocked for other commits.

This is a holdback from Dart 1 where some syntax or type errors were not
reported until runtime.

* prep for release

 Allow for multiple calls to executable.main()

- Fix some new implicit cast warnings.
- Adjust expectations for new source_span output.

In version `1.5.0` of `source_span` the output for multi-line spans was
updated to output the content and highlight a section with lines on the
left instead of the old `^^^^^` format. Update the expectations
accordingly to expect that a bit of the multi-line content is printed.

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

Fix DartDocs links

Also adjust the platform-specific-configuration links.