Python might have clout, but in many way it's even harder to optimize than JavaScript, whereas Dart was designed by JavaScript VM experts to be easily optimizable and has a very fast, very small VM.
Python is also only just starting to explore optional typing which is obviously a prime feature of Dart and JS derivatives like TypeScript.
It's very clear that this is not a technical problem, but a political one. Dart could be the second coming of Knuth and still nobody but Google would implement it in a major browser.
Python is also only just starting to explore optional typing which is obviously a prime feature of Dart and JS derivatives like TypeScript.