Fix up `AssemblyUtilities` to use `PortableShim`.

src/Compilers/Core/Portable/AssemblyUtilities.cs

@@ -58,7 +58,7 @@ public static ImmutableArray<string> FindAssemblySet(string filePath)
                         string referencePath = Path.Combine(directory, referenceName + ".dll");
 
                         if (!assemblySet.Contains(referencePath) &&
-                            File.Exists(referencePath))
+                            PortableShim.File.Exists(referencePath))
                         {
                             workList.Enqueue(referencePath);
                         }
@@ -108,16 +108,16 @@ public static ImmutableArray<string> FindSatelliteAssemblies(string filePath)
             string resourcesNameWithoutExtension = fileNameWithoutExtension + ".resources";
             string resourcesNameWithExtension = resourcesNameWithoutExtension + ".dll";
 
-            foreach (var subDirectory in Directory.EnumerateDirectories(directory))
+            foreach (var subDirectory in PortableShim.Directory.EnumerateDirectories(directory, "*", PortableShim.SearchOption.TopDirectoryOnly))
             {
                 string satelliteAssemblyPath = Path.Combine(subDirectory, resourcesNameWithExtension);
-                if (File.Exists(satelliteAssemblyPath))
+                if (PortableShim.File.Exists(satelliteAssemblyPath))
                 {
                     builder.Add(satelliteAssemblyPath);
                 }
 
                 satelliteAssemblyPath = Path.Combine(subDirectory, resourcesNameWithoutExtension, resourcesNameWithExtension);
-                if (File.Exists(satelliteAssemblyPath))
+                if (PortableShim.File.Exists(satelliteAssemblyPath))
                 {
                     builder.Add(satelliteAssemblyPath);
                 }

src/Compilers/Core/Portable/PortableShim.cs

@@ -188,10 +188,19 @@ internal static class Directory
                 contractName: $"{TypeName}, {System_IO_FileSystem_Name}",
                 desktopName: TypeName);
 
+            private static readonly MethodInfo s_enumerateDirectories = Type
+                .GetTypeInfo()
+                .GetDeclaredMethod(nameof(EnumerateDirectories), new[] { typeof(string), typeof(string), SearchOption.Type });
+
             private static readonly MethodInfo s_enumerateFiles = Type
                 .GetTypeInfo()
                 .GetDeclaredMethod(nameof(EnumerateFiles), new[] { typeof(string), typeof(string), SearchOption.Type });
 
+            internal static IEnumerable<string> EnumerateDirectories(string path, string searchPattern, object searchOption)
+            {
+                return (IEnumerable<string>)s_enumerateDirectories.Invoke(null, new object[] { path, searchPattern, searchOption });
+            }
+
             internal static IEnumerable<string> EnumerateFiles(string path, string searchPattern, object searchOption)
             {
                 return (IEnumerable<string>)s_enumerateFiles.Invoke(null, new object[] { path, searchPattern, searchOption });