Uses assembler macro for dart SP setup on arm64.

runtime/vm/stub_code_arm64.cc

Index: runtime/vm/stub_code_arm64.cc
===================================================================
--- runtime/vm/stub_code_arm64.cc	(revision 36930)
+++ runtime/vm/stub_code_arm64.cc	(working copy)
@@ -775,15 +775,19 @@
 //   R3 : new context containing the current isolate pointer.
 void StubCode::GenerateInvokeDartCodeStub(Assembler* assembler) {
   __ Comment("InvokeDartCodeStub");
+  // The new context, saved vm tag, the top exit frame, and the old context.
+  const intptr_t kNewContextOffsetFromFp =
+      -(1 + kAbiPreservedCpuRegCount + kAbiPreservedFpuRegCount) * kWordSize;
+
+  // Amount of space to reserve with the system stack pointer before setting it
+  // to the stack limit.
+  const intptr_t kRegSaveSpace = Utils::RoundUp(-kNewContextOffsetFromFp, 16);
+
   // Copy the C stack pointer (R31) into the stack pointer we'll actually use
   // to access the stack.
-  __ mov(SP, CSP);
+  __ SetupDartSP(kRegSaveSpace);
   __ EnterFrame(0);
 
-  // The new context, saved vm tag, the top exit frame, and the old context.
-  const intptr_t kNewContextOffsetFromFp =
-      -(1 + kAbiPreservedCpuRegCount + kAbiPreservedFpuRegCount) * kWordSize;
-
   // Save the callee-saved registers.
   for (int i = kAbiFirstPreservedCpuReg; i <= kAbiLastPreservedCpuReg; i++) {
     const Register r = static_cast<Register>(i);