Update .gitignore to new `dart_tool` pub cache

Allows clients to handle or rethrow errors instead of silently swallowing them.

Version 2.0.9 was already published with no changelog.

None of the planned breaking changes impact this package.

- Fix constraint style.
- Remove lower bound on `stack_trace` since versions before `1.0.0`
  didn't support the Dart 2 SDK anyway.
- Expand description.

Also fix deprecations in tests

Closes #30

dart-lang/sdk#32030

Remove usage of deprecated Dart 1 upper-case constants

This makes it easier to debug pending request errors.

Fix uses_dynamic_as_bottom error

* enable Travis-CI
* dartfmt
* Add analysis options

Fix broken tests

Closes #18

Closes #11

If the peer was closed and then the channel closed, we would
double-complete a completer.

R=jmesserly@google.com

Review URL: https://codereview.chromium.org//1890003002 .

This reverts commit 735d9b05 and
re-applies commit 078f891d.

It was determined that the build failure was caused by an upgraded SDK
version.

This reverts commit 078f891d, reversing
changes made to 21c97462.

This commit seemed to cause the bots to go red under mysterious
circumstances. Reverting to determine why.

Remove unused imports in the test files

Closes #8

R=tjblasi@google.com

Review URL: https://codereview.chromium.org//1780583002 .

It seems we tickled a core library inconsistency (dart-lang/sdk#25655):
although on the VM `FormatException.source` is set for JSON parse
errors, it is not set using dart2js.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1659313002 .

This converts the constructors to take StreamChannels, and changes some
edge-case semantics to be more familiar to StreamChannel (and WebSocket)
users.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1652413002 .