DevDiv #1089591: Handle null in MetadataUtilities.GetScopes

We shouldn't crash just because a PDB is missing information for a specific method.

It looks like the other callers of GetMethodByVersion handle null appropriately.

CR: ChuckS; KevinH (changeset 1387853)

Src/Test/PdbUtilities/Pdb/PdbToXml.cs

@@ -224,6 +224,7 @@ private void WriteMethod(MethodDefinitionHandle methodHandle)
                 // TODO (tomat): Ideally this would be done in a separate test helper, not in PdbToXml.
                 // verify ISymUnmanagedMethod APIs:
                 ISymUnmanagedMethod rawMethod = pdbReader.RawSymbolReader.GetBaselineMethod(token);
+                Debug.Assert(rawMethod != null, "How did we get an ISymbolMethod without a backing ISymUnmanagedMethod?");
 
                 var expectedSlotNames = new Dictionary<int, ImmutableArray<string>>();
                 WriteLocals(rawMethod, expectedSlotNames);

Src/Test/PdbUtilities/Shared/CustomDebugInfoReader.cs

@@ -396,7 +396,6 @@ public static ImmutableArray<ImmutableArray<string>> GetCSharpGroupedImportStrin
             var method = reader.GetMethodByVersion(methodToken, methodVersion);
             if (method == null)
             {
-                // TODO: remove this workaround for DevDiv #1060879.
                 return default(ImmutableArray<ImmutableArray<string>>);
             }
 
@@ -666,12 +665,8 @@ private static ImmutableArray<string> GetImportStrings(this ISymUnmanagedMethod
         {
             if (method == null)
             {
-                // TODO: remove this workaround for DevDiv #1060879.
-                // If methodToken was updated (because the method we started with forwards to another method),
-                // we have no way to know what version the new method is on.  If it's not the same as the
-                // version we stared with (e.g. because it has been changed less frequently), then we may not
-                // find a corresponding ISymUnmanagedMethod.
-                // Note: The real fix is to not use CDI forwarding in EnC PDBs.
+                // In rare circumstances (only bad PDBs?) GetMethodByVersion can return null.
+                // If there's no debug info for the method, then no import strings are available.
                 return ImmutableArray<string>.Empty;
             }