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

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 07cec18f5cdef130724ba156104bd701d27de44f..466226760d47f0e12d0319c496ecf6a57d39f48c 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -80,6 +80,9 @@ void HeapObject::HeapObjectVerify() {
   }
 
   switch (instance_type) {
+    case SYMBOL_TYPE:
+      Symbol::cast(this)->SymbolVerify();
+      break;
     case MAP_TYPE:
       Map::cast(this)->MapVerify();
       break;
@@ -213,6 +216,13 @@ void HeapObject::VerifyHeapPointer(Object* p) {
 }
 
 
+void Symbol::SymbolVerify() {
+  CHECK(IsSymbol());
+  CHECK(HasHashCode());
+  CHECK_GT(Hash(), 0);
+}
+
+
 void HeapNumber::HeapNumberVerify() {
   CHECK(IsHeapNumber());
 }