Require Dart 2 stable
web-angular: use stable dependencies

* [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.

- Move web-angular test into its own job. Fixes #424.
- Add an app version consistency check.
- Add a check that grind was run.

This runs the web-angular template component tests on Travis.

use containerized builds

Changed to just run the tests against stable.

Test against both the dev and stable SDKs.