Make instanceof and Object.getPrototypeOf work for proxies, plus a few other tweaks.

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 c2bc5acbd751ad1e692ccd66297d91a0e24f8739..076398906c9a63bb0d81325de8831f7834ee5daf 100644
--- a/src/x64/builtins-x64.cc
+++ b/src/x64/builtins-x64.cc
@@ -363,6 +363,7 @@ static void Generate_JSConstructStubHelper(MacroAssembler* masm,
 
   // If the type of the result (stored in its map) is less than
   // FIRST_SPEC_OBJECT_TYPE, it is not an object in the ECMA sense.
+  STATIC_ASSERT(LAST_SPEC_OBJECT_TYPE == LAST_TYPE);
   __ CmpObjectType(rax, FIRST_SPEC_OBJECT_TYPE, rcx);
   __ j(above_equal, &exit);