VM: Load allocation-top and -end via Thread.

runtime/vm/assembler_x64.h

Index: runtime/vm/assembler_x64.h
diff --git a/runtime/vm/assembler_x64.h b/runtime/vm/assembler_x64.h
index e0c68535f3d840980d0ab70b9a26ad829d5a7b51..3194083410a28c6a30974c88129c22c94019d85d 100644
--- a/runtime/vm/assembler_x64.h
+++ b/runtime/vm/assembler_x64.h
@@ -1013,14 +1013,16 @@ class Assembler : public ValueObject {
                    Label* failure,
                    bool near_jump,
                    Register instance_reg,
-                   Register pp);
+                   Register pp,
+                   Register temp);
 
   void TryAllocateArray(intptr_t cid,
                         intptr_t instance_size,
                         Label* failure,
                         bool near_jump,
                         Register instance,
-                        Register end_address);
+                        Register end_address,
+                        Register temp);
 
   // Debugging and bringup support.
   void Stop(const char* message, bool fixed_length_encoding = false);