If you don't use Dart-JS interop or any packages that rely on it, you can stop reading.
tl;dr: We are splitting up the current dart.js bootstrap script to move js-interop specific code to a separate file. If you use JS interop, you will also need to include interop.js alongside dart.js in your html file.
E.g., You will need to change your html from:
<script src="packages/browser/dart.js"></script>
to
<script src="packages/browser/dart.js"></script>
<script src="packages/browser/interop.js"></script>
Currently, dart.js includes a fair bit of code that's there only to support Dart-JS interop, and every Dart web app pays for this additional code regardless of whether they use JS interop or not.
I expect to land this change in the next 24 hours.
Cheers,
Vijay