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

runtime/vm/stub_code_arm64.cc

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_ARM64)
 
 #include "vm/assembler.h"
 #include "vm/code_generator.h"
 #include "vm/compiler.h"
@@ skipping 811 matching lines @@
   __ StoreToOffset(R6, THR, Thread::vm_tag_offset());
 
   // Save top resource and top exit frame info. Use R6 as a temporary register.
   // StackFrameIterator reads the top exit frame info saved in this frame.
   __ LoadFromOffset(R6, THR, Thread::top_resource_offset());
   __ StoreToOffset(ZR, THR, Thread::top_resource_offset());
   __ Push(R6);
   __ LoadFromOffset(R6, THR, Thread::top_exit_frame_info_offset());
   __ StoreToOffset(ZR, THR, Thread::top_exit_frame_info_offset());
   // kExitLinkSlotFromEntryFp must be kept in sync with the code below.
-  ASSERT(kExitLinkSlotFromEntryFp == -22);
+  ASSERT(kExitLinkSlotFromEntryFp == -21);
   __ Push(R6);
 
   // Load arguments descriptor array into R4, which is passed to Dart code.
   __ LoadFromOffset(R4, R1, VMHandles::kOffsetOfRawPtrInHandle);
 
   // Load number of arguments into S5.
   __ LoadFieldFromOffset(R5, R4, ArgumentsDescriptor::count_offset());
   __ SmiUntag(R5);
 
   // Compute address of 'arguments array' data area into R2.
@@ skipping 1302 matching lines @@
 // Result:
 //  R1: entry point.
 void StubCode::GenerateMegamorphicLookupStub(Assembler* assembler) {
   EmitMegamorphicLookup(assembler, R0, R1, R1);
   __ ret();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM64