Add data classes

Immutable data are used heavily for web applications today, commonly with Elm-like (redux, ngrx, ...) architectures. Most common thing web developer is doing with data is creating a copy of it with some fields changed, usually propagated to the root of state tree. JavaScript has spread operator for this. There should be a easy way to use immutable data structures in Dart. I would like to have data classes (inspired by [Kotlin]( https://kotlinlang.org/docs/reference/data-classes.html)) in Dart. Possible API:
```
data class User {
String name;
int age;
}
```
Compiler assumes that all fields of data class are immutable. Compiler adds `equals` implementation based on shallow equals, `hashCode` implementation based on mix of all object fields, `toString` implementation of the form ` =`, and `copy` function to recreate object with some fields changed.

You may argue that there is already [Built value](https://github.com/google/built_value.dart) package that allows to do similar things, but we have many issues with package, mainly:
1. It requires writing a lot of boilerplate code
2. It requires running watcher/manual code generation during development.
3. It requires saving generated files to repository because code generation time is too large for big applications.

I have found that using built value actually decreases my productivity and I do my work faster with even manually writing builders for classes.

If data classes would be implemented on language level, it would increase developer productivity and optimizations can be made when compiling code to particular platform.

No source file, test, or entry point is identified. Start by reading the issue discussion and the linked Kotlin data-classes reference, then compare the proposed API with the concerns about built_value.dart. Done would require an agreed, specified language-level data-class design rather than an isolated implementation.

Written by the indexing model from the issue text.