DeCo. Migrate everything to new AST.

No more flag, always parse into the new AST, always visit new AST nodes,
always return them as child entities, parent-child structure reflects
the new AST.

So, use `namePart` and `body` where possible. Deprecate previous
properties.

This is still de jure a breaking change, because `parent` of deprecated
properties changes. De facto this required very few changes in google3.

Once this CL lands, I will publish `analyzer 10.0.0`, migrate everything
to new properties, delete deprecated properties, and publish `analyzer
11.0.0`.

Maybe deprecate `NamedCompilationUnitMember.name` and migrate to
subclass specific `name` or `namePart` properties before publishing
`analyzer 10.0.0`. This part is not breaking per se.

* Deprecations in `ClassDeclaration`:
  * Properties `leftBracket`, `members`, `rightBracket` are deprecated, use `body` instead.
  * Properties `name`, `typeParameters` are deprecated, use `namePart` instead.
* Deprecations in `EnumDeclaration`:
  * Properties `leftBracket`, `constants`, `members`, `rightBracket` are deprecated, use `body` instead.
  * Properties `name`, `typeParameters` are deprecated, use `namePart` instead.
* Deprecations in `ExtensionDeclaration`:
  * Properties `leftBracket`, `members`, `rightBracket` are deprecated, use `body` instead.
* Deprecations in `ExtensionTypeDeclaration`:
  * Properties `leftBracket`, `constants`, `members`, `rightBracket` are deprecated, use `body` instead.
  * Properties `constKeyword`, `name`, `representation`, `typeParameters` are deprecated,
    use `primaryConstructor` instead.
* **Breaking Change:** While the deprecated members mentioned  above still exist in the AST,
  their parent nodes have changed. This means that code  relying on specific parent-child
  relationships for these nodes might break.

Bug: https://github.com/dart-lang/sdk/issues/61701
Change-Id: Ic48104da8b029c9b454bbd2336574b7823025565
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/461841
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>