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.

Start by tracing how the web debugger decodes VM-service base64 bytes for typed data instances and where it renders their values. Compare the required signed and unsigned 64-bit behavior, including Int64List and Uint64List, and consider the utility approaches mentioned; done means both types display meaningful values on the web.

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