Avoid compilation errors when the dart_target_arch is TARGET_ARCH_ARM

runtime/vm/cpu_arm.cc

Index: runtime/vm/cpu_arm.cc
diff --git a/runtime/vm/cpu_arm.cc b/runtime/vm/cpu_arm.cc
index 44964d279697eadc413592452f21c2d1543222f8..91d0364b3d2b76ad0456d414fd70c32ef4d93508 100644
--- a/runtime/vm/cpu_arm.cc
+++ b/runtime/vm/cpu_arm.cc
@@ -78,7 +78,12 @@ DEFINE_FLAG(bool, sim_use_hardfp, true, "Use the softfp ABI.");
 #endif
 
 void CPU::FlushICache(uword start, uword size) {
-#if !defined(USING_SIMULATOR)
+#if TARGET_OS_IOS
+  // Precompilation never patches code so there should be no I cache flushes.
+  UNREACHABLE();
+#endif
+
+#if !defined(USING_SIMULATOR) && !TARGET_OS_IOS
   // Nothing to do. Flushing no instructions.
   if (size == 0) {
     return;