Optimize AOT's switchable calls for the monomorphic case.

runtime/vm/assembler_arm64.h

Index: runtime/vm/assembler_arm64.h
diff --git a/runtime/vm/assembler_arm64.h b/runtime/vm/assembler_arm64.h
index c39e8e3261060ba40c545d02f31193024424f98f..1f8edfa2e89ea563abf417ceaeab2292ecdef7b9 100644
--- a/runtime/vm/assembler_arm64.h
+++ b/runtime/vm/assembler_arm64.h
@@ -488,7 +488,6 @@ class Assembler : public ValueObject {
   }
 
   void set_use_far_branches(bool b) {
-    ASSERT(buffer_.Size() == 0);
     use_far_branches_ = b;
   }
 
@@ -1364,6 +1363,9 @@ class Assembler : public ValueObject {
   void EnterStubFrame();
   void LeaveStubFrame();
 
+  void NoMonomorphicCheckedEntry();
+  void MonomorphicCheckedEntry();
+
   void UpdateAllocationStats(intptr_t cid,
                              Heap::Space space);