[ disclaimer: I work on the Dart team. ]
As timsneath mentioned above, Dartpad is the dev environment. That means we ship the full Dart and Flutter SDK ahead of time as you can type/change anything in the code box. We also ship debug metadata to give better errors.
In production mode, we do a much more expensive compile to remove the parts of Dart, Flutter, and packages you don't use and to optimize/minify the rest.
It is working in Safari for me, but we've definitely appreciate bug reports if it's breaking for you (https://github.com/dart-lang/dart-pad in this case).
* The page takes about 20s to load.
* After pressing Run, nothing happens for 10s, then Click me! text appears.
* On clicking the text, nothing happens.
* On clicking it a few times, the page crashes and reloads.