[pigeon] Update for compatibility with a future change to the analyzer. (flutter#3789)

We would like to make a change to the Dart analyzer, but the code in
pigeon is written in a way that is not compatible with the future
change. https://dart-review.googlesource.com/c/sdk/+/294920

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Author: scheglov

packages/pigeon/lib/pigeon_lib.dart

@@ -1071,15 +1071,14 @@ class _RootBuilder extends dart_ast_visitor.RecursiveAstVisitor<Object?> {
     if (_currentApi != null) {
       // Methods without named return types aren't supported.
       final dart_ast.TypeAnnotation returnType = node.returnType!;
-      final dart_ast.SimpleIdentifier returnTypeIdentifier =
-          getFirstChildOfType<dart_ast.SimpleIdentifier>(returnType)!;
+      returnType as dart_ast.NamedType;
       _currentApi!.methods.add(
         Method(
           name: node.name.lexeme,
           returnType: TypeDeclaration(
-              baseName: returnTypeIdentifier.name,
-              typeArguments: typeAnnotationsToTypeArguments(
-                  (returnType as dart_ast.NamedType).typeArguments),
+              baseName: returnType.name.name,
+              typeArguments:
+                  typeAnnotationsToTypeArguments(returnType.typeArguments),
               isNullable: returnType.question != null),
           arguments: arguments,
           isAsynchronous: isAsynchronous,