[WIP] Fable support update

Author: ncave

src/fsharp/TastOps.fs

@@ -5264,8 +5264,13 @@ let mkTyAppExpr m (f,fty) tyargs = match tyargs with [] -> f | _ -> primMkApp (f
 //--------------------------------------------------------------------------
 
 let rec accTargetsOfDecisionTree tree acc =
+#if FABLE_COMPILER
+  let f (opt: DecisionTree option) acc = if (Option.isSome opt) then (accTargetsOfDecisionTree opt.Value acc) else acc
+#else
+  let f (opt: DecisionTree option) acc = Option.foldBack accTargetsOfDecisionTree opt acc
+#endif
   match tree with 
-  | TDSwitch (_,edges,dflt,_) -> List.foldBack (fun (c:DecisionTreeCase) -> accTargetsOfDecisionTree c.CaseTree) edges (Option.foldBack accTargetsOfDecisionTree dflt acc)
+  | TDSwitch (_,edges,dflt,_) -> List.foldBack (fun (c:DecisionTreeCase) -> accTargetsOfDecisionTree c.CaseTree) edges (f dflt acc)
   | TDSuccess (_,i) -> i::acc
   | TDBind (_,rest) -> accTargetsOfDecisionTree rest acc