[dart2js] Use futureValueType in impact computation.

Change-Id: Ic6f54cf14098c9cc1cfe01ee01a09e7be5ce419a
Bug: #54317
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/341600
Reviewed-by: Nate Biggs <[email protected]>
Commit-Queue: Mayank Patke <[email protected]>

Author: fishythefish

pkg/compiler/lib/src/ir/impact_data.dart

@@ -185,14 +185,7 @@ class ImpactBuilder extends StaticTypeVisitor implements ImpactRegistry {
         break;
 
       case ir.AsyncMarker.Async:
-        ir.DartType elementType = const ir.DynamicType();
-        if (returnType is ir.InterfaceType &&
-            returnType.classNode == typeEnvironment.coreTypes.futureClass) {
-          elementType = returnType.typeArguments.first;
-        } else if (returnType is ir.FutureOrType) {
-          elementType = returnType.typeArgument;
-        }
-        registerAsync(elementType);
+        registerAsync(function.futureValueType!);
         break;
 
       case ir.AsyncMarker.AsyncStar: