"Delete the bin/js-debug folder and try again." That did the trick. I have to get used to this new approach. With Flex since is a single file, is always updated. There was probably old files from the previous compile with the previous SDK version.
Now I can see the button on the page and without any console errors however when I press the button, nothing happens and I get the following console error: Alert.js:159 Uncaught TypeError: Cannot read property 'systemManager' of undefined at Function.mx.controls.Alert.show (Alert.js:159) at HelloRoyale.$EH0 (HelloRoyale.mxml:12) at Function.goog.events.fireListener [as googFireListener] (events.js:744) at Object.org.apache.royale.core.HTMLElementWrapper.fireListenerOverride [as fireListener] (HTMLElementWrapper.js:61) at HTMLButtonElement.goog.events.handleBrowserEvent_ (events.js:870) at HTMLButtonElement.f (events.js:289) -- Sent from: http://apache-royale-users.20374.n8.nabble.com/