Don't touch x18 on ARM64 - it is globally reserved on iOS.

runtime/vm/constants_x64.h

Index: runtime/vm/constants_x64.h
diff --git a/runtime/vm/constants_x64.h b/runtime/vm/constants_x64.h
index c2b48c56cab5b89b08ad3ae8eef6c0ef4765ffbd..9c92faa5d030af33954b91ee4b929e957dfb0385 100644
--- a/runtime/vm/constants_x64.h
+++ b/runtime/vm/constants_x64.h
@@ -8,7 +8,6 @@
 namespace dart {
 
 enum Register {
-  kFirstFreeCpuRegister = 0,
   RAX = 0,
   RCX = 1,
   RDX = 2,
@@ -25,9 +24,8 @@ enum Register {
   R13 = 13,
   R14 = 14,
   R15 = 15,
-  kLastFreeCpuRegister = 15,
   kNumberOfCpuRegisters = 16,
-  kNoRegister = -1  // Signals an illegal register.
+  kNoRegister = -1,  // Signals an illegal register.
 };
 
 
@@ -96,6 +94,7 @@ const Register ARGS_DESC_REG = R10;  // Arguments descriptor register.
 const Register CODE_REG = R12;
 const Register THR = R14;  // Caches current thread in generated code.
 
+
 // Exception object is passed in this register to the catch handlers when an
 // exception is thrown.
 const Register kExceptionObjectReg = RAX;
@@ -105,6 +104,20 @@ const Register kExceptionObjectReg = RAX;
 const Register kStackTraceObjectReg = RDX;
 
 
+typedef uint32_t RegList;
+const RegList kAllCpuRegistersList = 0xFFFF;
+
+const RegList kReservedCpuRegisters =
+    (1 << SPREG) |
+    (1 << FPREG) |
+    (1 << TMP)   |
+    (1 << PP)    |
+    (1 << THR);
+// CPU registers available to Dart allocator.
+const RegList kDartAvailableCpuRegs =
+    kAllCpuRegistersList & ~kReservedCpuRegisters;
+
+
 enum ScaleFactor {
   TIMES_1 = 0,
   TIMES_2 = 1,