Updating Observatory docs to 1.5.

Updating to 1.5.

Merge branch 'master' into cloud

Adding cloud page.

Tweaking.

Merge branch 'master' into mobile

Incorporating feedback.

Merge branch 'master' into mobile

Incorporating comment from Keerti.

Merge branch 'master' into mobile

Adding new screenshot.

Dart Editor has changed how this works. I have updated the docs accordingly.

Merge branch 'master' into mobile

Tweaking

Merge branch 'master' into mobile

Tweaks

Tweaking.

BUG=
R=johnmccutchan@google.com

Review URL: https://codereview.appspot.com/104940044