WebKit is supporting Python bindings but that language is 3rd after Objective-C and Apple pays the big Obj-C costs.
My comment about Dart's merits was purely a businessman's evaluation, not NIH. Before you throw that accusation my way, consider Mozilla using Google WebM code, and lately implementing SPDY.
You expect other competing browsers to give Google a strategic free lunch, and to give up their share of influence in standards bodies? No way, not from us, or from Apple or Microsoft or Opera.
But it turns out Dart does not have obvious merit, such that I am moved to implement it (see SPDY for a counter-example). Dart is jejune, knee-jerk, and conservative in its design, from what I have now seen.