ES6 symbols: Implement Symbol intrinsic and basic functionality

src/x64/stub-cache-x64.cc

Index: src/x64/stub-cache-x64.cc
diff --git a/src/x64/stub-cache-x64.cc b/src/x64/stub-cache-x64.cc
index 23c444f467aa0c4123ea268eb8b7f8febacb54c5..4be5738756ea9a663c0bac5cc89ade52dbeae834 100644
--- a/src/x64/stub-cache-x64.cc
+++ b/src/x64/stub-cache-x64.cc
@@ -2262,6 +2262,12 @@ Handle<Code> CallStubCompiler::CompileCallConstant(Handle<Object> object,
       }
       break;
 
+    case SYMBOL_CHECK:
+      // Check that the object is a symbol.
+      __ CmpObjectType(rdx, SYMBOL_TYPE, rax);
+      __ j(not_equal, &miss);
+      break;
+
     case NUMBER_CHECK:
       if (function->IsBuiltin() || !function->shared()->is_classic_mode()) {
         Label fast;