Fix sharing coded buffer bytes when parsing bytes fields

[osa1]

Synced from internal repo

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I'm not sure if this is the right fix for this bug. Should readBytes() return a new list (instead of a view) maybe?

[sigurdm]

LGTM

[osa1]

I'm not sure if this is the right fix for this bug. Should readBytes() return a new list (instead of a view) maybe?

We discussed this fix with @sigurdm. Some notes about
CodedBufferReader.readBytes:

• It's supposed to be internal, but it's currently part of the public API. This
  is probably accidental.

• In the library we have 4 uses of this method, and with this PR all of them
  clone the returned list, either explicitly with Uint8List.fromList, or by
  decoding the returned list to a string, or appending returned list to another
  list.

• Internally (in g3) we have a few users, they all explicitly copy the returned
  list.

• So in principle we could make CodedBufferReader.readBytes return a new list
  (instead of a view).

• It's also not unreasonable to make proto messages share the buffer, but we
  need to do one of the following:

  • Document it so that the users will clone the buffer to avoid sharing (note
    that proto messages are not length prefixed, and I don't know how difficult
    or inconvenient this would be in practice)
  • Implement copy-on-write so that backing buffer will never be modified