Optimize AOT's switchable calls for the monomorphic case.

runtime/vm/assembler_arm.h

Index: runtime/vm/assembler_arm.h
diff --git a/runtime/vm/assembler_arm.h b/runtime/vm/assembler_arm.h
index 184f28d4829bf06c3d82462640005579eaefcde0..3afce467b61f7fa12f5548d6367788fc6397702c 100644
--- a/runtime/vm/assembler_arm.h
+++ b/runtime/vm/assembler_arm.h
@@ -368,7 +368,9 @@ class Assembler : public ValueObject {
         prologue_offset_(-1),
         use_far_branches_(use_far_branches),
         comments_(),
-        constant_pool_allowed_(false) { }
+        constant_pool_allowed_(false) {
+    MonomorphicCheckedEntry();
+  }
 
   ~Assembler() { }
 
@@ -944,6 +946,9 @@ class Assembler : public ValueObject {
   void EnterStubFrame();
   void LeaveStubFrame();
 
+  void NoMonomorphicCheckedEntry();
+  void MonomorphicCheckedEntry();
+
   // The register into which the allocation stats table is loaded with
   // LoadAllocationStatsAddress should be passed to
   // IncrementAllocationStats(WithSize) as stats_addr_reg to update the