Since ImmutableDictionary and ImmutableSortedDictionary are both
implemented via trees; you can think of an ImmutableDictionary as just
being a sorted dictionary where the underlying key is the hash int.
Since our hash is just a hash of a GUID, we can compare by the GUID,
switch to a ImmutableSortedDictionary, and then avoid another sort
when we're computing checksums.

We previously were using ImmutableArrays, so if you had a large number
of documents (say thousands) we'd be creating huge numbers of huge
arrays over and over again. For this pattern, ImmutableList is more
appropriate.

* Add and implement IScriptEnvironmentService

* Remove unnecessary export

Use frozen-partial semantics when producing 'accurate' tags.  That way we won't block users as much when copying code.

If we're going to require it, let's at least crash with a useful
exception instead of a null ref.

Revert "Merge pull request #20558 from Pilchie/Fix19968-DuplicateSourceAndAdditionalFiles-DistinctIds"

This reverts commit 6f5b2d47, reversing
changes made to 66362df5.

Fixes #20599

This is a breaking API change.  Need to decide if we want to take it or
not.

basically code will think tree is already serialized to storage when it actually never did which cause us to crash.

CompilationTracker.HasSuccessfullyLoadedAsync had a same bug with 60cceb9f

Fixes #19570

https://github.com/dotnet/roslyn/pull/19620#discussion_r117289958

we found 2 issues. 1. hard to validate persisted data and 2. recalculating checksum is faster than reading from storage.