Optimize AOT's switchable calls for the monomorphic case.

runtime/vm/stack_frame.cc

Index: runtime/vm/stack_frame.cc
diff --git a/runtime/vm/stack_frame.cc b/runtime/vm/stack_frame.cc
index d9bd0fa679b842e82ac03a27489b2428e3f79dae..f93111e41e411e7a8fd1075898fd605590c2ea1e 100644
--- a/runtime/vm/stack_frame.cc
+++ b/runtime/vm/stack_frame.cc
@@ -108,9 +108,8 @@ void StackFrame::VisitObjectPointers(ObjectPointerVisitor* visitor) {
     Array maps;
     maps = Array::null();
     Stackmap map;
-    const uword entry = reinterpret_cast<uword>(code.instructions()->ptr()) +
-                        Instructions::HeaderSize();
-    map = code.GetStackmap(pc() - entry, &maps, &map);
+    const uword start = Instructions::PayloadStart(code.instructions());
+    map = code.GetStackmap(pc() - start, &maps, &map);
     if (!map.IsNull()) {
 #if !defined(TARGET_ARCH_DBC)
       RawObject** first = reinterpret_cast<RawObject**>(sp());
@@ -260,7 +259,7 @@ bool StackFrame::FindExceptionHandler(Thread* thread,
   if (code.IsNull()) {
     return false;  // Stub frames do not have exception handlers.
   }
-  uword pc_offset = pc() - code.EntryPoint();
+  uword pc_offset = pc() - code.PayloadStart();
 
   REUSABLE_EXCEPTION_HANDLERS_HANDLESCOPE(thread);
   ExceptionHandlers& handlers = reused_exception_handlers_handle.Handle();
@@ -279,7 +278,7 @@ bool StackFrame::FindExceptionHandler(Thread* thread,
     if ((iter.PcOffset() == pc_offset) && (current_try_index != -1)) {
       RawExceptionHandlers::HandlerInfo handler_info;
       handlers.GetHandlerInfo(current_try_index, &handler_info);
-      *handler_pc = code.EntryPoint() + handler_info.handler_pc_offset;
+      *handler_pc = code.PayloadStart() + handler_info.handler_pc_offset;
       *needs_stacktrace = handler_info.needs_stacktrace;
       *has_catch_all = handler_info.has_catch_all;
       return true;
@@ -294,7 +293,7 @@ TokenPosition StackFrame::GetTokenPos() const {
   if (code.IsNull()) {
     return TokenPosition::kNoSource;  // Stub frames do not have token_pos.
   }
-  uword pc_offset = pc() - code.EntryPoint();
+  uword pc_offset = pc() - code.PayloadStart();
   const PcDescriptors& descriptors =
       PcDescriptors::Handle(code.pc_descriptors());
   ASSERT(!descriptors.IsNull());