chore(pubspec): Replace shadow_dom with web_components

[vicb]

The shadow_dom package is deprecated, see
https://code.google.com/p/dart/issues/detail?id=18080

see also #956 (comment)

[vicb]

The tests seem to fail. One possible reason is that the scripts should be added to the head section which is currently not possible with karma, see karma-runner/karma#488

[mhevery]

This change breaks FireFox dart2js tests. Could you look into it.

[pavelgj]

There's also http://dartbug.com/18747 --- Angular.dart tests failing in Firefox with exactly the same errors.

[vicb]

Thanks for the ref @pavelgj, I'll look at it next week

[vicb]

Current status, failing on Dart VM version: 1.4.0-dev.6.2 (Thu May 8 05:18:01 2014) on "linux_x64"

Firefox 30.0.0 (Ubuntu): Executed 1458 of 1458 (70 FAILED)
Chrome 36.0.1976 (Linux): Executed 1458 of 1458 SUCCESS

Firefox errors are:

Firefox 30.0.0 (Ubuntu) introspection should select elements using CSS selector FAILED
    Test failed: Caught Instance of 'UnknownJavaScriptObject'   

It does work with Dart VM version: 1.3.6 (Tue Apr 29 12:40:24 2014) on "linux_x64"

[jbdeboer]

https://code.google.com/p/dart/issues/detail?id=18747 was fixed, but we are still failing.

https://travis-ci.org/angular/angular.dart/jobs/25354277

@pavelgj Is there something more that we need to do?

[jbdeboer]

Ah, I forgot to upgrade web_components to 0.3.4

[jbdeboer]

Here is a fun one:
Dart 1.3 does not support web_components 0.3.4
Dart 1.4 does not support web_components 0.3.3
Angular supports them both.

So, I set pubspec.yaml to be web_components >=0.3.3 and set pubspec.lock to be web_components 0.3.4

pub get in Dart 1.4 will take 0.3.4 from the lock file, but Dart 1.3 will automatically downgrade to 0.3.3