Unfortunately no built-in support for source code (HTML) editing isn't provided by CKEditor and "Source" button simply doesn't work. Luckily the SourceDialog add-on (see http://ckeditor.com/addon/sourcedialog) exists, that adds such ability. When installed however it's UI looks broken and doesn't match one on developer website:
|Expected UI||Actual UI|
Turns out the cause of this is presense of "CodeMirror" addon alongside with "SourceDialog" addon.
Disable "CodeMirror" CKEditor add-on, when using CKEditor for inline content block editing.