Uses an object pool on x64

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
===================================================================
--- runtime/vm/assembler_x64.h	(revision 26436)
+++ runtime/vm/assembler_x64.h	(working copy)
@@ -342,7 +342,7 @@
   void call(Label* label);
   void call(const ExternalLabel* label);
 
-  static const intptr_t kCallExternalLabelSize = 13;
+  static const intptr_t kCallExternalLabelSize = 10;
 
   void pushq(Register reg);
   void pushq(const Address& address);
@@ -652,7 +652,12 @@
 
   void Drop(intptr_t stack_elements);
 
+  int32_t AddObject(const Object& obj);
+  int32_t AddExternalLabel(const ExternalLabel* label);
+  void LoadWordFromPoolOffset(Register dst, int32_t offset);
+  void LoadExternalLabel(const ExternalLabel* label);
   void LoadObject(Register dst, const Object& object);
+  void CallPatchable(const ExternalLabel* label);
   void StoreObject(const Address& dst, const Object& obj);
   void PushObject(const Object& object);
   void CompareObject(Register reg, const Object& object);
@@ -679,7 +684,7 @@
   }
 
   void EnterFrame(intptr_t frame_space);
-  void LeaveFrame();
+  void LeaveFrame(bool restore_pp = false);
   void ReserveAlignedFrameSpace(intptr_t frame_space);
 
   // Create a frame for calling into runtime that preserves all volatile
@@ -731,6 +736,7 @@
     buffer_.FinalizeInstructions(region);
   }
 
+  void LoadPoolPointer();
   // Set up a Dart frame on entry with a frame pointer and PC information to
   // enable easy access to the RawInstruction object of code corresponding
   // to this frame.
@@ -739,6 +745,7 @@
   //   ret PC
   //   saved RBP     <=== RBP
   //   pc (used to derive the RawInstruction Object of the dart code)
+  //   saved PP
   //   locals space  <=== RSP
   //   .....
   // This code sets this up with the sequence:
@@ -746,6 +753,8 @@
   //   movq rbp, rsp
   //   call L
   //   L: <code to adjust saved pc if there is any intrinsification code>
+  //   ...
+  //   pushq r15
   //   .....
   void EnterDartFrame(intptr_t frame_size);
 
@@ -767,9 +776,9 @@
   //   movq rbp, rsp
   //   pushq immediate(0)
   //   .....
-  void EnterStubFrame();
+  void EnterStubFrame(bool save_pp = false);
 
-  // Instruction pattern from entrypoint is used in dart frame prologs
+  // Instruction pattern from entrypoint is used in dart frame prologues
   // to set up the frame and save a PC which can be used to figure out the
   // RawInstruction object corresponding to the code running in the frame.
   // entrypoint:
@@ -802,7 +811,7 @@
 
  private:
   AssemblerBuffer buffer_;
-  GrowableObjectArray& object_pool_;  // Object pool is not used on x64.
+  GrowableObjectArray& object_pool_;  // Objects and patchable jump targets.
   int prologue_offset_;
 
   class CodeComment : public ZoneAllocated {