ES6 symbols: Implement Symbol intrinsic and basic functionality

src/x64/builtins-x64.cc

Index: src/x64/builtins-x64.cc
diff --git a/src/x64/builtins-x64.cc b/src/x64/builtins-x64.cc
index 5d87ee795bfa6a159171d506f5b6006d19d6e46b..f2f31aa8fea1ef09b5f21ae3feec03907f2d5413 100644
--- a/src/x64/builtins-x64.cc
+++ b/src/x64/builtins-x64.cc
@@ -389,6 +389,10 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
     __ CmpObjectType(rax, FIRST_SPEC_OBJECT_TYPE, rcx);
     __ j(above_equal, &exit);
 
+    // Symbols are "objects".
+    __ CmpInstanceType(rcx, SYMBOL_TYPE);
+    __ j(equal, &exit);
+
     // Throw away the result of the constructor invocation and use the
     // on-stack receiver as the result.
     __ bind(&use_receiver);