Fix most strong mode warnings.

lib/src/result/future.dart

Index: lib/src/result/future.dart
diff --git a/lib/src/result/future.dart b/lib/src/result/future.dart
index db9dd822c112883fe93d7ab08279c1b0c7939a8d..209e8b1e6b226fe5a9d51d579f143f088082c25f 100644
--- a/lib/src/result/future.dart
+++ b/lib/src/result/future.dart
@@ -20,11 +20,12 @@ class ResultFuture<T> extends DelegatingFuture<T> {
   Result<T> _result;
 
   factory ResultFuture(Future<T> future) {
-    var resultFuture;
-    resultFuture = new ResultFuture._(Result.capture(future).then((result) {
+    ResultFuture<T> resultFuture;
+    resultFuture = new ResultFuture._(() async {
+      var result = await Result.capture(future);
       resultFuture._result = result;
-      return result.asFuture;
-    }));
+      return await result.asFuture;
+    }());
     return resultFuture;
   }