Fix places awaiting void, for dart 2 (#393)

Author: MichaelRFairhurst

lib/src/async_environment.dart

@@ -339,7 +339,7 @@ class AsyncEnvironment {
   }
 
   /// Sets [inMixin] to `true` for the duration of [callback].
-  Future asMixin(void callback()) async {
+  Future asMixin(Future callback()) async {
     var oldInMixin = _inMixin;
     _inMixin = true;
     await callback();

lib/src/environment.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_environment.dart.
 // See tool/synchronize.dart for details.
 //
-// Checksum: 09ec65a907dc0d104f8d65be803428dd99fcb1c8
+// Checksum: 5489295cd8e8ae8e80deb3b5941285263863b15f
 
 import 'package:source_span/source_span.dart';
 

test/cli/shared/source_maps.dart

@@ -278,7 +278,7 @@ void sharedTests(Future<TestProcess> runSass(Iterable<String> arguments)) {
 
     group("with the target in a different directory", () {
       setUp(() async {
-        await ensureDir(d.path("dir"));
+        ensureDir(d.path("dir"));
         await (await runSass(
                 ["--embed-source-map", "test.scss", "dir/out.css"]))
             .shouldExit(0);