[js] Make nqp::cleardispatcher behave like on the MoarVM

Author: pmurias

src/vm/js/Operations.nqp

@@ -1691,7 +1691,7 @@ class QAST::OperationsJS {
             ~ "\}\n"
         );
     });
-    add_simple_op('cleardispatcher', $T_VOID, [], sub () {"nqp.currentDispatcher = undefined"}, :side_effects);
+    add_simple_op('cleardispatcher', $T_VOID, [], :ctx, :side_effects);
     add_simple_op('setdispatcher', $T_VOID, [$T_OBJ], sub ($value) {"nqp.currentDispatcher = $value"}, :side_effects);
     add_simple_op('setdispatcherfor', $T_VOID, [$T_OBJ, $T_OBJ], :side_effects);
 

src/vm/js/nqp-runtime/core.js

@@ -1587,6 +1587,12 @@ op.unicmp_s = function(a, b, collationMode, language, code) {
   return (result < 0 ? -1 : (result > 0 ? 1 : 0));
 };
 
+op.cleardispatcher = function(ctx) {
+  if (nqp.currentDispatcherFor === ctx.codeRef() || nqp.currentDispatcherFor === undefined) {
+    nqp.currentDispatcher = undefined
+  }
+};
+
 const getrusage = require('qrusage');
 op.getrusage = function() {
   const usage = getrusage();