Freeze messages during parsing to avoid freeze pass

Currently if I use a message in read-only way I need to first parse it, then do a pass to freeze it.

If we had a way of generating frozen (read-only) messages in parsers that would eliminate the extra pass for freezing.

Implementation should be trivial, but extending the generated message API in a backwards-compatible way will be sub-optimal because we can't mix optional positional and named arguments (https://github.com/dart-lang/language/issues/1076).

If mixing positional and named optionals were allowed we could extend the `fromBuffer` and other parse methods with an optional named argument `{bool freeze = false}`. Since this is not possible yet we will need to add a new constructor like `fromBufferReadOnly` (or `fromBufferFrozen`, and another one for parsing JSON). With luck these extra constructors will be tree-shaken when not used and won't cause binary size issues in JS and AOT.

I'm looking at internal users to see if freezing right after parsing is a common pattern.

Start by reviewing the generated message parsing entry points, especially fromBuffer and the JSON parsing methods, then inspect the internal users mentioned in the issue for freeze-after-parse patterns. Done means parsers can produce frozen messages without a separate freezing pass while preserving backwards compatibility.

Written by the indexing model from the issue text.