Proposal: support unboxed type.

Now dart is a real statically typed language(beginning from Dart 2.0) and being used for develop mobile apps. Flutter is a native performance cross-platform framework. To better complete with native apps(written in java/kotlin/swift), high performance is important.

So, this feature request intends to support unboxed type(something like [ java value type, inline classes](https://cr.openjdk.java.net/~briangoetz/valhalla/sov/01-background.html)[1], value types without object identity) to further improve performance and reduce memory usage.

_[1]State of Valhalla. The Road to Valhalla(https://cr.openjdk.java.net/~briangoetz/valhalla/sov/01-background.html)_

Start by reading the linked State of Valhalla background on Java value types and inline classes. Then define how Dart would support unboxed types without object identity, including the performance and memory goals stated in the issue. The issue does not name files, tests, entry points, or acceptance criteria.

Written by the indexing model from the issue text.