Google recognises this issue. An internal email from Nov 2010 that outlines their strategy for 'Dart', says:
"Complex web apps--the kind that Google specializes in--are struggling against the platform and working with a language that cannot be tooled and has inherent performance problems"
That's quite an ambituous goal!
With so many people looking to use HTML5 as part of their future web strategy and as more complex applications are built on the web, Dart could be a good thing for all of us.
Regards, Colin E.