[vm] Add type to _AsyncAwaitCompleter.start.

Without the type, the call to the the parameter f() was being handled
as a dynamic InstanceCall (smiable), so we had a megamorphic call to one
of the Closure_XXXX_call methods (there are many copies of this method
for different signatures) which in turn calls the closure.

Now that the type is present, we do a direct ClosureCall to the target.

This gives a measurable improvement to an important internal benchmark.

Change-Id: Ia9429879f69257465a2537616d8f8595fb1eefa8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138520
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Samir Jindel <sjindel@google.com>

sdk/lib/_internal/vm/lib/async_patch.dart

@@ -43,7 +43,7 @@ class _AsyncAwaitCompleter<T> implements Completer<T> {
     }
   }
 
-  void start(f) {
+  void start(void Function() f) {
     f();
     isSync = true;
   }

sdk_nnbd/lib/_internal/vm/lib/async_patch.dart

@@ -41,7 +41,7 @@ class _AsyncAwaitCompleter<T> implements Completer<T> {
     }
   }
 
-  void start(f) {
+  void start(void Function() f) {
     f();
     isSync = true;
   }