Immutable types

[dmdashenkov]

In this PR we introduce immutable Dart types generated based on Protobuf definitions.

Standard Protobuf Dart tool generated code which is always mutable. Even though messages can be "frozen", their API always has setters. This is confusing so we want to make all Protobuf messages immutable by default.\

The current idea of going about this problem is to generate abstract classes based on Protobuf definitions and then feed those classes to built_value — a builder which would complete the boilerplate side of code generation, such as builders, null checks, etc.

[alexander-yevsyukov]

@dmdashenkov, have a look at https://pub.dev/packages/freezed. Their approach does not require calling a method. They construct things already immutable.

[dmdashenkov]

Suspended for now. Please do not delete the branch.

[alexander-yevsyukov]

@dmdashenkov, I'm reopening the PR to:

1. prevent accidental deletion of the associated branch;
2. make the fact that we've got work in progress and ongoing communications with Google visible to the rest of the team and people getting familiar with the framework.

Please also mention the issue you created in this PR, so that people can find it from that issue and see why our request is important.

[alexander-yevsyukov]

@dmdashenkov, please also describe this PR so that people can quickly grasp what it's all about.

[alexander-yevsyukov]

@dmdashenkov, what you've had in meeting minutes with @armiol should go into this PR.

[dmdashenkov]

While developing this feature, we've faced a couple of problems.

1. Standard Protobuf Dart API provides no common interface for each message which we would be able to leverage. This means that there's no way to maintain the structure of the message or to use polymorphism in our client API.
2. Standard messages have to be retained in public access scope, since we want to use them for serialization purposes. If we hide the messages into a file-private scope, they become unavailable for other messages to be used as field types, which is unacceptable. We're left with 2 sets of classes: mutable and immutable, which are separates in names with a class name prefix.

[dmdashenkov]

Right now, we're trying to find out if the Dart team could help us with this problem. Please see the issue.