At the moment (prior to version SAPUI5 1.28.4), styleClass is an unsupported property of sap.m.Button and its base type ( sap.ui.core.Control ). Therefore, you must use addStyleClass(sStyleClass) OR in the XML view directly.
As @Ivan said, you can use the busy property since it exists in the base type sap.ui.core.Control
We hope that this basic functionality will be available in higher versions.
Update: for multiple CSS classes
var oLabel = new sap.m.Label({text:"Sample"}).addStyleClass("sample1 sample2");
OR
var oLabel = new sap.m.Label({text:"Sample"}).addStyleClass("sample1").addStyleClass("sample2");
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