Allow debugger to step into Map and Set forEach callbacks

src/collection.js

Index: src/collection.js
diff --git a/src/collection.js b/src/collection.js
index fefc7a0499db3c71ea1585ab2eb5a8767af755f9..cf70e06545912d3153ab15452c24e1d457c92d94 100644
--- a/src/collection.js
+++ b/src/collection.js
@@ -106,7 +106,9 @@ function SetForEach(f, receiver) {
 
   var iterator = %SetCreateIterator(this, ITERATOR_KIND_VALUES);
   var entry;
+  var stepping = %_DebugCallbackSupportsStepping(f);
   while (!(entry = %SetIteratorNext(iterator)).done) {
+    if (stepping) %DebugPrepareStepInIfStepping(f);
     %_CallFunction(receiver, entry.value, entry.value, this, f);
   }
 }
@@ -215,7 +217,9 @@ function MapForEach(f, receiver) {
 
   var iterator = %MapCreateIterator(this, ITERATOR_KIND_ENTRIES);
   var entry;
+  var stepping = %_DebugCallbackSupportsStepping(f);
   while (!(entry = %MapIteratorNext(iterator)).done) {
+    if (stepping) %DebugPrepareStepInIfStepping(f);
     %_CallFunction(receiver, entry.value[1], entry.value[0], this, f);
   }
 }