Web version of devtools cannot display 64-bit numbers from typed data instances

For typed data instances, the VM service will return the byte contents encoded as a base64 String. In order to display a meaningful value in the debugger, this must be decoded and then converted to the appropriate typed data instance. For example, -1 will need to display as `0xFF` if the typed data instance was unsigned.

This poses a problem for `Unit64List` and `int64List`, since these types cannot be instantiated on the web.

One potential solution would be to pull in [fixnum](https://github.com/dart-lang/fixnum) and manually unmarshal Int64 numbers into a string representation - though that would not help the unsigned case. Another approach would be to write a utility based off of fixnum to handle both.

首先跟踪 Web 调试器如何为类型化数据实例解码 VM-service 的 base64 字节，以及它在哪里渲染这些值。比较有符号和无符号 64 位值所需的行为，包括 Int64List 和 Uint64List，并考虑提到的实用工具方案；完成的标准是这两种类型都能在 Web 上显示有意义的值。

由索引模型根据 Issue 内容生成。