Use ConcurrentDictionary in ImportMap (#4148)

src/fsharp/import.fs

@@ -4,6 +4,7 @@
 module internal Microsoft.FSharp.Compiler.Import
 
 open System.Reflection
+open System.Collections.Concurrent
 open System.Collections.Generic
 
 open Microsoft.FSharp.Compiler.AbstractIL.IL
@@ -52,7 +53,7 @@ type AssemblyLoader =
 /// serves as an interface through to the tables stored in the primary TcImports structures defined in CompileOps.fs. 
 [<Sealed>]
 type ImportMap(g:TcGlobals,assemblyLoader:AssemblyLoader) =
-    let typeRefToTyconRefCache = new System.Collections.Generic.Dictionary<ILTypeRef,TyconRef>()
+    let typeRefToTyconRefCache = ConcurrentDictionary<ILTypeRef,TyconRef>()
     member this.g = g
     member this.assemblyLoader = assemblyLoader
     member this.ILTypeRefToTyconRefCache = typeRefToTyconRefCache