[vm,aot,bytecode] Do not depend on presence of _AsyncAwaitCompleter

In AOT mode tree shaker can remove _AsyncAwaitCompleter class, so
bytecode generator should not crash if it can't find this class in
the core library.

The only use of _AsyncAwaitCompleter in bytecode generator is to
mark _AsyncAwaitCompleter.get:future as non-debuggable.

This change fixes crash when building Flutter engine in
host/release configuration with unreachable code elimination
(https://dart-review.googlesource.com/c/sdk/+/121901).

Change-Id: I7724ef6ae51b2b2039bb113c5c7328956bf6e99d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/122167
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>

Author: alexmarkov

pkg/vm/lib/bytecode/gen_bytecode.dart

@@ -1071,7 +1071,7 @@ class BytecodeGenerator extends RecursiveVisitor<Null> {
 
   Procedure _asyncAwaitCompleterGetFuture;
   Procedure get asyncAwaitCompleterGetFuture =>
-      _asyncAwaitCompleterGetFuture ??= libraryIndex.getMember(
+      _asyncAwaitCompleterGetFuture ??= libraryIndex.tryGetMember(
           'dart:async', '_AsyncAwaitCompleter', 'get:future');
 
   Procedure _setAsyncThreadStackTrace;