Parsing of protos from a buffer is slow when the proto / buffer is large

An internal 1p application uses protos in the following manner at startup. `buffer` is obtained from some sort of persistent storage.

```dart
defaultInstance.rebuild((message) => message.mergeFromBuffer(buffer));
```

A rough benchmark shows that this is slow for protos that are "large" in both the size of the buffer, and the number of fields. For example, some buffers that are 10KB in size can lead to 20ms of parsing time on a low power Android device.

See b/229065886 for more details, along with some CPU profiles.

Start by profiling mergeFromBuffer with large buffers and many fields, using the startup pattern described in the issue and the CPU profiles referenced in b/229065886. Done means identifying and addressing the parsing bottleneck, then showing improved parsing time for comparable large-protobuf cases.

Written by the indexing model from the issue text.