Opera, latest version that is 12 or higher.That's a goal; we don't actually support all of these browsers yet. The goal may change to be either more restrictive or more permissive. We'll refine this further as Dart matures.Q. What browsers support Dart now?
As of February 2012, no production browsers can execute Dart code unless it's first compiled to JavaScript. However, a Chromium build (nicknamed Dartium) is available that can execute Dart code natively. For details on downloading and using Dartium, see Chromium with the Dart VM.
Q. How do I debug Dart code after it's been compiled to JavaScript?
The generated JavaScript from dart2js is readable, and it's reasonable to use the browser's development tools on it. We expect to eventually provide Source Maps output, which allows you to debug the Dart code from the JavaScript code inside of your browser's developer tools.
Yes, we intend for any valid Dart code to compile to JavaScript. If some support is missing from one of our compilers, that's a bug (in either the compiler or the spec).
Q. Does Dart support JSON?
Yes, Dart libraries can parse from and stringify to JSON. You can find such a library atdart/lib/json/
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Q. Will Dart run on the server?
We have some server code now, such as our test suites. Have a look at the file, socket, and process APIs in dart/runtime/bin/
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更新正在进行的Google IO Dart session的一个截屏:

更新最后问答阶段的一些内容:
1、dart编译成js的时候,是针对每个browser编译一份,还是编译一份js可以在多种browser上运行?
答:编译一份js
2、dart editor可否变成Eclipse的plugin,这样开发者就可以使用Eclipse的种种工具和好处?
答:日期还不能透露,但最终肯定会支持Eclipse plugin的形式
3、什么是dart成功的标志?
答:在chrome内置dart虚拟机,完美地支持dart,在google之外的很多项目决定使用dart,dart逐渐被整个web开发社区认可,这就是dart成功的标志,也是dart team的ultimate goal。
4、dart对server端的支持如何?
答:dart很好地支持json,所以client端和server端可以以json的形式传输数据,dart虚拟机也可以在server端运行,dart是一个支持在client端和server端都可以运行的语言。
笔者认为,google毫无疑问会大力发展和推广dart,chrome已经决定内置dart虚拟机,现在最大的疑问就是其他主流浏览器是否愿意直接支持dart,尤其是mozilla firefox,如果dart本身只能在chrome一个浏览器上运行,在其他浏览器上都必须先编译成javascript,那它的优越性就大大减弱了(试想一下,dart是一门语言,javascript也是一门语言,对于一个不懂javascript的人来说,学习javascript固然是痛苦的,学习dart再编译成javascript也没有吸引力)。相反,如果mozilla也内置dart虚拟机,那dart就会掀起一场web系统开发的革命,浏览器的效率将得到革命性的提升!结合cloud技术,未来的终端机真的只要browser+memory就可以了!