MetadataLoadContext to support dynamic assemblies

[steveharter]

As part of the review for feature #15033 and PR dotnet/corefx#33201, there was concern about the lack of support for assemblies not backed by a physical file.

From the comments @jkotas:

There is no way to load runtime loaded assembly into MetadataLoadContext that it not backed by a physical file. The runtime has AssemblyExtensions.TryGetRawMetadata API to handle this case, but there is no way to give the blob returned by TryGetRawMetadata to MetadataLoadContext. The assemblies not backed by physical file are rare today, but they are very likely going to become more common as part of the single-file project.
I am wondering whether there should be a LoadFrom method that takes MetadataReaderProvider to address both these problems and/or whether we should expose a resolver that can resolve the runtime loaded assemblies for you.

Implementation\design TBD. One approach is to use runtime reflection over the dynamic assembly, and convert those from System.RuntimeType to the MetadataLoadContext types (System.RoType).

[steveharter]

Moving to future.

[steveharter]

There have been no customer asks for this so far, although the scenario is valid.

Due to 5.0 schedule constraints, this is being moved to Future.

[slang25]

I would really like support for this 🙂. I want to load some dynamic assemblies that are built in memory with S.R.Emit, and then load them into MetadataLoadContext for unit testing my load context reflection logic.