Upgrade Symbol implementation to match current ES6 behavior.

src/x64/code-stubs-x64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2252 matching lines @@
     if (RecordCallTarget()) {
       GenerateRecordCallTarget(masm);
     }
   }
 
   // Fast-case: Just invoke the function.
   ParameterCount actual(argc_);
 
   if (CallAsMethod()) {
     if (NeedsChecks()) {
+      // FIXME: expedient but expensive; surely a better way?
+      // Always call ToObject() on symbol values.
+      Label not_a_symbol;
+      __ movp(rax, Operand(rsp, (argc_ + 1) * kPointerSize));
+      __ JumpIfSmi(rax, &not_a_symbol, Label::kNear);
+      __ CmpObjectType(rax, SYMBOL_TYPE, rcx);
+      __ j(equal, &wrap);
+
+      __ bind(&not_a_symbol);
       // Do not transform the receiver for strict mode functions.
       __ movp(rcx, FieldOperand(rdi, JSFunction::kSharedFunctionInfoOffset));
       __ testb(FieldOperand(rcx, SharedFunctionInfo::kStrictModeByteOffset),
                Immediate(1 << SharedFunctionInfo::kStrictModeBitWithinByte));
       __ j(not_equal, &cont);
 
       // Do not transform the receiver for natives.
       // SharedFunctionInfo is already loaded into rcx.
       __ testb(FieldOperand(rcx, SharedFunctionInfo::kNativeByteOffset),
                Immediate(1 << SharedFunctionInfo::kNativeBitWithinByte));
@@ skipping 3011 matching lines @@
                               return_value_operand,
                               NULL);
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_X64