Is Microsoft planning to have C# compile down to javascript code so that C# code can run in the browser? Pretty much like what I understand Dart does?

Just read Dart does not support Tuples. https://groups.google.com/a/dartlang.org/forum/?fromgroups#!topic/misc/yih2CeeSBnI

and no extension methods, at least as of Oct, 2011: http://code.google.com/p/dart/issues/detail?id=13