[dart2wasm] Improve dispatch table packing from 49% to 99%

The most important thing with dispatch table layout is to order classes
in depth-first pre-order numbers and make it only include concrete
classes.

* We only dispatch on concrete classes.
* Any selector of a base class is inherited by its subclasses
* Using DFS makes selector rows have few but wide filled blocks.

This CL orders class ids in a pure DFS pre-order numbering:

* DFS pre-order traversal for concrete classes
* DFS pre-order traversal for abstract classes

=> Increases dispatch table packing from 49% to 99%
=> Reduces GDT from 31k entries to 15k entries on Flute
=> Reduces Flute by around 2.1% code size.

It also prepares for implementing faster type checks by doing class-id
range checks.

Change-Id: I96744b114e8815904f7405dca1432cdca3c7fe30
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/348561
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>