I recompiled my code -without changing aything- with the 2.0-m2, and both setIconStyle and addStyleName are working.
So I confirm that this seems to be a bug in the 2.0-m3 version.
If I'm wrong, tell me why.
This is what I'm doing :
And whatever I put in my CSS, the excel-button class isn't applied to the button :
Button excelButton = new Button();
ToolBar toolbar = new ToolBar();
<button class="x-btn-text" ...>