Track the 'awaiter return' call stack use it to detect uncaught exceptions in async functions

runtime/vm/flow_graph.h

Index: runtime/vm/flow_graph.h
diff --git a/runtime/vm/flow_graph.h b/runtime/vm/flow_graph.h
index 9c1c08e2b295640945be08162110a88adcfe37cc..40fb247fa656245e0c2a8976a004d8e5aa44f2f2 100644
--- a/runtime/vm/flow_graph.h
+++ b/runtime/vm/flow_graph.h
@@ -285,6 +285,15 @@ class FlowGraph : public ZoneAllocated {
   // Merge instructions (only per basic-block).
   void TryOptimizePatterns();
 
+  ZoneGrowableArray<TokenPosition>* await_token_positions() const {
+    return await_token_positions_;
+  }
+
+  void set_await_token_positions(
+      ZoneGrowableArray<TokenPosition>* await_token_positions) {
+    await_token_positions_ = await_token_positions;
+  }
+
   // Replaces uses that are dominated by dom of 'def' with 'other'.
   // Note: uses that occur at instruction dom itself are not dominated by it.
   static void RenameDominatedUses(Definition* def,
@@ -386,6 +395,7 @@ class FlowGraph : public ZoneAllocated {
   ZoneGrowableArray<BlockEntryInstr*>* loop_headers_;
   ZoneGrowableArray<BitVector*>* loop_invariant_loads_;
   ZoneGrowableArray<const LibraryPrefix*>* deferred_prefixes_;
+  ZoneGrowableArray<TokenPosition>* await_token_positions_;
   DirectChainedHashMap<ConstantPoolTrait> constant_instr_pool_;
   BitVector* captured_parameters_;