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

runtime/vm/simulator_arm64.cc

Index: runtime/vm/simulator_arm64.cc
diff --git a/runtime/vm/simulator_arm64.cc b/runtime/vm/simulator_arm64.cc
index ac6fe18793d7ac90b766cb5dfc224e8604cf30a6..eca82f33b1be779908b99fb0bf4c00a15b45bc8e 100644
--- a/runtime/vm/simulator_arm64.cc
+++ b/runtime/vm/simulator_arm64.cc
@@ -900,6 +900,7 @@ void Simulator::set_register(
     Instr* instr, Register reg, int64_t value, R31Type r31t) {
   // Register is in range.
   ASSERT((reg >= 0) && (reg < kNumberOfCpuRegisters));
+  ASSERT(instr == NULL || reg != R18);  // R18 is globally reserved on iOS.
   if ((reg != R31) || (r31t != R31IsZR)) {
     registers_[reg] = value;
     // If we're setting CSP, make sure it is 16-byte aligned. In truth, CSP
@@ -1666,7 +1667,7 @@ void Simulator::DoRedirectedCall(Instr* instr) {
     set_register(instr, R15, icount_);
     set_register(instr, IP0, icount_);
     set_register(instr, IP1, icount_);
-    set_register(instr, R18, icount_);
+    set_register(NULL, R18, icount_);
     set_register(instr, LR, icount_);
 
     // TODO(zra): Zap caller-saved fpu registers.