Convert to async function body refactoring should handle FutureOr return types

[pattobrien]

Description

Given a return type of FutureOr<Foo>, the code assist Convert to async function body improperly adds another Future to the return type, resulting in a new return type of Future<FutureOr<Foo>>.

Before assist:

@override
  FutureOr<void> buildDefinitionForMethod(
    MethodDeclaration method,
    FunctionDefinitionBuilder builder,
  ) { /* ... */ }

After assist:

@override
  Future<FutureOr<void>> buildDefinitionForMethod(
    MethodDeclaration method,
    FunctionDefinitionBuilder builder,
  ) async { /* ... */ }

Info

Dart SDK version: 3.5.0-159.0.dev (dev) (Wed May 15 13:03:12 2024 -0700) on "macos_arm64"