ES6 symbols: Introduce Symbol class, along with abstract Name class

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index da922bd3d52835417932eae581a3aa490a292129..9d92d673e6b044dd1397fced1cd6c5d09033f0a1 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -87,6 +87,7 @@ namespace internal {
   V(FixedArray, regexp_multiple_cache, RegExpMultipleCache)                    \
   V(Object, termination_exception, TerminationException)                       \
   V(Smi, hash_seed, HashSeed)                                                  \
+  V(Map, symbol_map, SymbolMap)                                                \
   V(Map, string_map, StringMap)                                                \
   V(Map, ascii_string_map, AsciiStringMap)                                     \
   V(Map, cons_string_map, ConsStringMap)                                       \
@@ -841,6 +842,13 @@ class Heap {
       void* external_pointer,
       PretenureFlag pretenure);
 
+  // Allocate a symbol.
+  // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
+  // failed.
+  // Please note this does not perform a garbage collection.
+  MUST_USE_RESULT MaybeObject* AllocateSymbol(
+      PretenureFlag pretenure = NOT_TENURED);
+
   // Allocate a tenured JS global property cell.
   // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
   // failed.