Google introduces Dart, do we need a JavaScript alternative

Google has introduced Dart, a new programming language it hopes to meet the following goals:

• "Create a structured yet flexible language for web programming.
• Make Dart feel familiar and natural to programmers and thus easy to learn.
• Ensure that Dart delivers high performance on all modern web browsers and environments ranging from small handheld devices to server-side execution.

Dart: a language for structured web programming - The official Google Code blog:

It seem kind of late in the game to try an introduce an alternative to JavaScript.  This would be far from the first effort to replace JavaScript with another web client logic, and to date JavaScript has gone from triumph to triumph.  HTML 5 has just been released and it is probably better to to see how that evolves rather than learn a new alternative unproven technology.

Microsoft did not have much luck with SilverLight which tried to do the same thing.