Some symbols don't resolve that should

@stephentoub and I were looking at MemoryExtensions.Count. For example: https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs,4009

You cannot search for this method, but you can navigate to it from the class name. When clicking on the name of the member it does nothing: https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs,b6c053206fec6870

The method body does also not resolve the called method.

Perhaps roslyn is having difficulty with syntax in this file (eg: we need a newer roslyn in the indexer?) or maybe some other problem is making it not possible to index. I tried to find logs mentioning any problems with these types from the index build but didn't see anything. Maybe we could try opening the stage1 solution to see if there's an issue.