[builtins] Start refactoring the Apply builtin.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index d0a0dabf9e4a8c8c4160ce8b2537c3c4c9623c6f..b976eb971bc862bf0d448bb884d321c3acac8ee4 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -1149,6 +1149,9 @@ class MacroAssembler: public Assembler {
   void AssertSmi(Register object);
   void AssertSmi(const Operand& object);
 
+  // Abort execution if argument is not a FixedArray, enabled via --debug-code.
+  void AssertFixedArray(Register object);
+
   // Abort execution if a 64 bit register containing a 32 bit payload does not
   // have zeros in the top 32 bits, enabled via --debug-code.
   void AssertZeroExtended(Register reg);