(disclaimer, I work with the Dart team)
However, we feel the web needs more complex, more performant apps (eg more apps on the size of Google Docs). With user expectations shifting to apps, we feel the web needs to remain competitive to the demands of modern app developers. We'd like to help keep the web a compelling platform for developers from all platforms, like Android, iOS, and .NET. Those developers have a different set of expectations of their tools, languages, and platform. Dart is a project that aims to be a solid choice for developers building more structured web apps.
Hope that helps,