If Chrome DevTools and Dart DevTools are opened at the same time, both show a debugger overlay

[elliette]

The "Paused in Dart DevTools" overlay was added in #1476

Unfortunately, if both Chrome DevTools and Dart DevTools are open at the same time, both of their overlays are added to the page (the Chrome DevTools overlay is added above the Dart overlay). We should figure out how to detect if Chrome DevTools is open and therefore don't show the message. A quick fix would be to shorten the wording so that the Dart overlay remains hidden beneath the Chrome overlay.