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

runtime/vm/constants_ia32.h

Index: runtime/vm/constants_ia32.h
diff --git a/runtime/vm/constants_ia32.h b/runtime/vm/constants_ia32.h
index 0950639ad51355cb4de4596c8b2ed5ae7a944ebb..466dd42b520519fdadad76a36bb501094dc5c396 100644
--- a/runtime/vm/constants_ia32.h
+++ b/runtime/vm/constants_ia32.h
@@ -10,7 +10,6 @@
 namespace dart {
 
 enum Register {
-  kFirstFreeCpuRegister = 0,
   EAX = 0,
   ECX = 1,
   EDX = 2,
@@ -19,9 +18,8 @@ enum Register {
   EBP = 5,
   ESI = 6,
   EDI = 7,
-  kLastFreeCpuRegister = 7,
   kNumberOfCpuRegisters = 8,
-  kNoRegister = -1  // Signals an illegal register.
+  kNoRegister = -1,  // Signals an illegal register.
 };
 
 
@@ -71,6 +69,7 @@ const Register ICREG = ECX;  // IC data register.
 const Register ARGS_DESC_REG = EDX;  // Arguments descriptor register.
 const Register THR = ESI;  // 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 = EAX;
@@ -80,6 +79,18 @@ const Register kExceptionObjectReg = EAX;
 const Register kStackTraceObjectReg = EDX;
 
 
+typedef uint32_t RegList;
+const RegList kAllCpuRegistersList = 0xFF;
+
+const intptr_t kReservedCpuRegisters =
+    (1 << SPREG) |
+    (1 << FPREG) |
+    (1 << THR);
+// CPU registers available to Dart allocator.
+const RegList kDartAvailableCpuRegs =
+    kAllCpuRegistersList & ~kReservedCpuRegisters;
+
+
 enum ScaleFactor {
   TIMES_1 = 0,
   TIMES_2 = 1,