Fix code awaiting void values, for dart 2.

pkgs/test_descriptor/lib/src/file_descriptor.dart

diff --git a/pkgs/test_descriptor/lib/src/file_descriptor.dart b/pkgs/test_descriptor/lib/src/file_descriptor.dart
index ef030f0..93dec58 100644
--- a/pkgs/test_descriptor/lib/src/file_descriptor.dart
+++ b/pkgs/test_descriptor/lib/src/file_descriptor.dart
@@ -85,7 +85,7 @@
   ///
   /// The [prettyPath] is a human-friendly representation of the path to the
   /// descriptor.
-  void _validate(String prettyPath, List<int> binaryContents);
+  Future _validate(String prettyPath, List<int> binaryContents);
 
   /// Reads and decodes the contents of this descriptor as a UTF-8 string.
   ///
@@ -184,7 +184,7 @@
   Stream<List<int>> readAsBytes() =>
       throw new UnsupportedError("Matcher files can't be created or read.");
 
-  void _validate(String prettyPath, List<int> actualContents) {
+  Future _validate(String prettyPath, List<int> actualContents) async {
     try {
       expect(
           _isBinary ? actualContents : utf8.decode(actualContents), _matcher);