AST. Preserve token types of names in serialized ASTs.

Names in serialized ASTs were read back as `TokenType.STRING` tokens.
The V1 projection decides between `PrefixedIdentifier` and
`PropertyAccess` by the token type of the name, so `A.zero` in a
constant initializer projected as `PrefixedIdentifier` when the library
was linked, but as `PropertyAccess` when it was read from a bundle.

Now the type of each name token is written together with its lexeme,
and the reader restores a token of the same type, a `KeywordToken` for
keywords. So keyword names, such as `A.when`, and reserved words kept
by recovery, such as `A.class`, also round-trip.

The token type mapping was missing `augment`, `base`, `extension`,
`sealed`, `when`, the literal types with digit separators, `BAD_INPUT`,
and `RECOVERY`, so these are added, with a test that every token type
can be serialized. Synthetic name tokens built by `ElementBuilder` are
now `TokenType.IDENTIFIER`, so that a round trip doesn't restore them as
string literal tokens.

The SDK summary grows by 0.24%; stable_analysis instructions: local +0.009%, Golem presubmit +0.17%.

Change-Id: Ida01b1aa5d43041042efc4151914126f5fb3c0e2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/568801
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>