Require Dart 2 stable
web-angular: use stable dependencies

Fixes #568

Also:

* Update version to 3.1.1
* Temporary disable Travis job that uses dart_coveralls

Contributes to #561.

Fixes #547

Fixes #542

Fixes #530
Fixes #535

* Upgrade min SDK and web-angular

Fixes #506, #521, #522

* web-angular: add link to testing docs

Fixes #519

Fixes https://github.com/dart-lang/stagehand/issues/513

Fixes https://github.com/dart-lang/stagehand/issues/508

...and prepare to release 2.0.1

Closes https://github.com/dart-lang/stagehand/issues/470

* Remove gh-page task – not used any more
* update min SDK for all templates

* Update server-shelf
* Update all web projects to the min build_* package webdev versions
* Update all packages min SDK

Part of https://github.com/dart-lang/stagehand/issues/475

* Support the latest pkg:args
* Standardize authors to align with other Dart packages
* stagexl: Removed used of pkg:browser, pkg:dart_to_js_script_rewriter

Progress towards https://github.com/dart-lang/stagehand/issues/472

* [2-dev] Update min SDK; add Angular analyzer plugin, etc

- Set minimal SDK to 2.0.0-dev.28.0
- Enabled Angular analyzer plugin
- Replace builder config file with new named ones.
- Switch to mock `pageloader`
- `CORE_DIRECTIVES` -> `coreDirectives`
- Dropped <del>`const`</del> qualifier from literals used used in metadata annotations
- Switched to using new provider classes like `ClassProvider`

* Generated files

* Dart 2 changes: constant variable renames, etc

* Re-enable Angular tests over web-angular template

* Pin to build_runner to 0.8.0

* Test script tweaks since we can't yet use chrome headless

* Travis: use sudo required

* Revert build_runner dependency_overrides

* Use jsonDecode and tweak travis.sh

* Update to Dart 2, and Angular 5-alpha for web-angular
* Adjusted test/validate_templates.dart
* Allow build-check to fail since we won't commit generated files.

* Fix version check script

* Add .dart_tool to .gitignore files x

Fixes #459

* Generated files

Followup to #427

Also improved the CHANGELOG

A better example of proper usage, too – IMHO

Closes https://github.com/google/stagehand/issues/425

* Remove platform* options.

* Update CHANGELOG.