Work around dart-lang/sdk#33574

lib/src/importer/node/implementation.dart

@@ -9,12 +9,11 @@ import 'package:path/path.dart' as p;
 import 'package:tuple/tuple.dart';
 
 import '../../io.dart';
+import '../../node/function.dart';
 import '../../node/importer_result.dart';
 import '../../node/utils.dart';
 import '../utils.dart';
 
-typedef _Importer(String url, String prev, [void done(result)]);
-
 /// An importer that encapsulates Node Sass's import logic.
 ///
 /// This isn't a normal [Importer] because Node Sass's import behavior isn't
@@ -46,10 +45,9 @@ class NodeImporter {
   final List<String> _includePaths;
 
   /// The importer functions passed in by the user.
-  final List<_Importer> _importers;
+  final List<JSFunction> _importers;
 
-  NodeImporter(this._context, Iterable<String> includePaths,
-      Iterable<_Importer> importers)
+  NodeImporter(this._context, Iterable<String> includePaths, Iterable importers)
       : _includePaths = new List.unmodifiable(includePaths),
         _importers = new List.unmodifiable(importers);
 
@@ -137,8 +135,8 @@ class NodeImporter {
         : new Tuple2(readFile(resolved), p.toUri(resolved).toString());
   }
 
-  /// Converts an [_Importer]'s return [value] to a tuple that can be returned
-  /// by [load].
+  /// Converts an importer's return [value] to a tuple that can be returned by
+  /// [load].
   Tuple2<String, String> _handleImportResult(
       String url, Uri previous, Object value) {
     if (isJSError(value)) throw value;
@@ -164,7 +162,7 @@ class NodeImporter {
 
   /// Calls an importer that may or may not be asynchronous.
   Future<Object> _callImporterAsync(
-      _Importer importer, String url, String previousString) async {
+      JSFunction importer, String url, String previousString) async {
     var completer = new Completer();
 
     var result = call3(importer, _context, url, previousString,

lib/src/importer/node/interface.dart

@@ -6,11 +6,9 @@ import 'dart:async';
 
 import 'package:tuple/tuple.dart';
 
-typedef _Importer(String url, String prev, [void done(result)]);
-
 class NodeImporter {
-  NodeImporter(Object context, Iterable<String> includePaths,
-      Iterable<_Importer> importers);
+  NodeImporter(
+      Object context, Iterable<String> includePaths, Iterable importers);
 
   Tuple2<String, String> load(String url, Uri previous) => null;
 

lib/src/node.dart

@@ -19,6 +19,7 @@ import 'executable.dart' as executable;
 import 'importer/node.dart';
 import 'node/error.dart';
 import 'node/exports.dart';
+import 'node/function.dart';
 import 'node/render_context.dart';
 import 'node/render_context_options.dart';
 import 'node/render_options.dart';
@@ -30,8 +31,6 @@ import 'parse/scss.dart';
 import 'value.dart';
 import 'visitor/serialize.dart';
 
-typedef _Importer(String url, String prev, [void done(result)]);
-
 /// The entrypoint for Node.js.
 ///
 /// This sets up exports that can be called from JS. These include a private
@@ -251,13 +250,13 @@ List<AsyncCallable> _parseFunctions(RenderOptions options,
 /// Parses [importer] and [includePaths] from [RenderOptions] into a
 /// [NodeImporter].
 NodeImporter _parseImporter(RenderOptions options, DateTime start) {
-  List<_Importer> importers;
+  List<JSFunction> importers;
   if (options.importer == null) {
     importers = [];
   } else if (options.importer is List) {
     importers = (options.importer as List).cast();
   } else {
-    importers = [options.importer as _Importer];
+    importers = [options.importer as JSFunction];
   }
 
   var includePaths = options.includePaths ?? [];
@@ -294,7 +293,7 @@ NodeImporter _parseImporter(RenderOptions options, DateTime start) {
         }));
         if (isUndefined(result)) return options.fiber.yield();
         return result;
-      }) as _Importer;
+      }) as JSFunction;
     }).toList();
   }
 

lib/src/node/function.dart

@@ -5,7 +5,7 @@
 import 'package:js/js.dart';
 
 @JS("Function")
-class JSFunction {
+class JSFunction implements Function {
   external JSFunction(String arg1, [String arg2, String arg3]);
 
   // Note that this just invokes the function with the given arguments, rather

lib/src/node/utils.dart

@@ -40,14 +40,13 @@ external Function get _JSError;
 bool isJSError(value) => instanceof(value, _JSError);
 
 /// Invokes [function] with [thisArg] as `this`.
-R call2<R, A1, A2>(
-        R function(A1 arg1, A2 arg2), Object thisArg, A1 arg1, A2 arg2) =>
-    (function as JSFunction).apply(thisArg, [arg1, arg2]) as R;
+Object call2(JSFunction function, Object thisArg, Object arg1, Object arg2) =>
+    function.apply(thisArg, [arg1, arg2]);
 
 /// Invokes [function] with [thisArg] as `this`.
-R call3<R, A1, A2, A3>(R function(A1 arg1, A2 arg2, A3 arg3), Object thisArg,
-        A1 arg1, A2 arg2, A3 arg3) =>
-    (function as JSFunction).apply(thisArg, [arg1, arg2, arg3]) as R;
+Object call3(JSFunction function, Object thisArg, Object arg1, Object arg2,
+        Object arg3) =>
+    function.apply(thisArg, [arg1, arg2, arg3]);
 
 @JS("Object.keys")
 external List<String> _keys(Object object);