- cl/660721933: moves some proto3 parsing functions to utils to allow reusing
  internally in a library when parsing an internal format.

- cl/646759129: Update `CodedBufferReader` default depth limit to 100, to be
  consistent with the C++ and Java implementations.

- cl/704280814: Update `CodedBufferReader` default size limit to max 32-bit
  signed integer to be consistent with the C++, Java, Objective-C
  implementations.

- Also a minor change, not a CL: Type casts in `coded_buffer_writer.dart` was
  updated in this repo probably accidentally, as we never want to type cast
  with `as` in this repo. Revert it to the internal version which uses implicit
  casts.

With this, about half of the protobuf files become identical with the internal
version.

When an extension field set isn't initialized before freezing the message:

- `addExtension` allows adding and modifying extensions.
- `getExtension` returns non-frozen defaults that can be modified. 

Fix this by initializing the extension field set as frozen when the parent
message is frozen.

Secondly, extension field set returns differently typed lists when it's frozen
and not frozen. Fix this by always creating the list via the field's
`FieldInfo`.

Also refactor the code a little bit: `GeneratedMessage` by-passes a layer of
abstraction and accesses unknown field sets directly. Instead make it call
`FieldSet` always.

New `FieldSet` methods are added as public (without an underscore in the name)
to make it clear that they're a public interface (can be called from
`GeneratedMessage`).

(Ideally `FieldSet` should be a separate library, but that requires a lot of
refactoring to split the parts into libraries.)

Also remove duplicate "is repeated" and "ensure writable" checks when modifying
extension fields.

---

cl/819146715

A while ago we started converting parts to libraries to allow conditional
compilation.

This continues to move more parts to libraries, with `PbList`.

Libraries allow specifying public and internally public interfaces of the types
and makes it easier to define and maintain internal invariants.

To avoid large (difficult to sync) diffs, I'm moving one part at a time.

apitool reports no changes with this PR.

Mutation testing discovered that removing the `_ensureWritable` line in
`FieldSet.clearExtension` doesn't cause any test failures. Add a test that
fails when the line is removed.

Continuation to #1063.

apitool reports no changes with this PR.

Map decoding code moved from `pb_map.dart` to `coded_buffer.dart` to avoid
accessing `CodedBufferReader` internals in `PbMap`. It's also more consistent
to have all decoding code in `coded_buffer.dart`. We don't have repeated field
decoding code in `pb_list.dart`, and `pb_list.dart` and `pb_map.dart` don't
have decoders for other formats.

Static members `_keyFieldNumber` and `_valueFieldNumber` are removed as they
were duplicates of top-level members `mapKeyFieldNumber` and
`mapValueFieldNumber`.

`deepCopy` is exported as an internal extension to avoid expanding the public
API without consideration.

For {extension,non-extension} {singular,repeated} fields, check range and type
checks.

All fields with range checks are tested. For type and nullability checks we
only use a message and a `bool` field, as there are too many types of fields
and most of them use the same validation function.

Fields with range checks are not tested for type and nullability checks. To
check whether a value is within range we have to use as a non-null `int` or
`double`, so if the value being checked doesn't have the right type the range
check function will throw anyway.

This reveals two inconsistencies in the library:

- When type of a non-repeated extension value is not right we throw an argument
  error instead of a type error.

- `setExtension` takes an `Object` (not nullable) as an argument but
  `addExtension` takes `Object?` (nullable).

I tested accordingly instead of trying to fix the inconsistencies as fixing
them will break downstream.

Since sound null safety we no longer need to manually check for nulls when
setting fields, as we don't use nullable types for field values.

Fixes #978.

`createRepeated` members are not that useful, because `PbList` doesn't provide
much over the standard library `List`, and `MyMessage.createRepeated()` can be
replaced with `<MyMessage>[]`.

To reduce generated code sizes and dependencies to `PbList`, remove the
`createRepeated` members.

Update `PbList` methods that update the list with an `Iterable` argument to
avoid iterating the iterable twice: once for checking the element validity and
once again for actually adding the values.

Methods updated:

- addAll
- insertAll
- replaceRange
- setAll
- setRange

Exception handling behavior before this PR was undefined (same as the standard
library `List`), and it's slightly changed with this PR:

- addAll: previously if the iterator throws the list was left unchanged, now
  the elements until the exception will be added.

- Other methods: exception behaviors are now the same as the standard library
  `List` methods.

  It's hard to tell whether the previous behavior was the same or different
  with the standard library `List` methods, as the exception behavior of those
  are undefined.

To avoid allocating new iterators when a check function is not available, we
have conditionals in each of these methods and call the standard library `List`
methods directly when there isn't a check function.

To avoid increasing number of cases needed to be tested, we don't special case
iterable types like `PbList` and `List` in these methods. When the check
function is available we simply `map` them with the check function. Otherwise
call the same method on `wrappedList` directly.

Fixes #730.