Added chrome.tabs APIs.
Merge remote-tracking branch 'upstream/master'
Added test_tabs.dart.

Updated names of typedefs in windows.dart and tabs.dart to prevent name
conflicts.

Implemented Window.tabs getter.
Added chrome.browserAction APIs and tests.
Add chrome.contextMenus APIs.
Make names more consistent.

Add sendMessage and onMessage methods to Runtime.

Change Runtime to match style conventions used elsewhere (non-static members on class, global singleton).
Fix errors caught by dartanalyzer.
Merge pull request #52 from DrMarcII/master

More chrome extension APIs.