Better support for 'polymorphic' JS and external arrays

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 0e46253af98e2ed67ffe80cee1548873d88fb701..d7d5af3bd33e704bc2c07291c33edef6d8b0fa70 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -45,6 +45,7 @@ enum AllocationFlags {
   RESULT_CONTAINS_TOP = 1 << 1
 };
 
+
 // Default scratch register used by MacroAssembler (and other code that needs
 // a spare register). The register isn't callee save, and not used by the
 // function calling convention.
@@ -752,6 +753,14 @@ class MacroAssembler: public Assembler {
                 Label* fail,
                 SmiCheckType smi_check_type);
 
+  // Check if the map of an object is equal to a specified map and branch to a
+  // specified target if equal. Skip the smi check if not required (object is
+  // known to be a heap object)
+  void DispatchMap(Register obj,
+                   Handle<Map> map,
+                   Handle<Code> success,
+                   SmiCheckType smi_check_type);
+
   // Check if the object in register heap_object is a string. Afterwards the
   // register map contains the object map and the register instance_type
   // contains the instance_type. The registers map and instance_type can be the