More HttpClientResponse Uint8List fixes (#35143)
More HttpClientResponse Uint8List fixes (#35143)

Follow-on change to https://github.com/flutter/flutter/pull/34863 (see that change
for context), whereby we ensure that we're properly dealing in `Uint8List`.

These necessary changes would have been caught by disabling implicit casts
in our analysis options.

dart-lang/sdk#36900
https://github.com/flutter/flutter/issues/13815
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