Improve JSON stringifier's escape check.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 8cef1aea5394f07db103f61e639695339bcb8343..d28b3e449b343b0761a3a371d44511fcfc3ff0a6 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -2398,6 +2398,12 @@ bool Heap::CreateInitialMaps() {
     roots_[entry.index] = Map::cast(obj);
   }
 
+  { MaybeObject* maybe_obj = AllocateMap(ASCII_SYMBOL_TYPE,
+                                         kVariableSizeSentinel);
+    if (!maybe_obj->ToObject(&obj)) return false;
+  }
+  set_printable_ascii_symbol_map(Map::cast(obj));
+
   { MaybeObject* maybe_obj = AllocateMap(STRING_TYPE, kVariableSizeSentinel);
     if (!maybe_obj->ToObject(&obj)) return false;
   }