Add expectAsyncX and expectAsyncUntilX methods, and deprecate the old methods.

test/util/one_off_handler_test.dart

Index: test/util/one_off_handler_test.dart
diff --git a/test/util/one_off_handler_test.dart b/test/util/one_off_handler_test.dart
index 44c2e4cbbce96678092cfd8905c52eebab15eeda..af7db225662c0f2feaee003290a1d95a6e5593ff 100644
--- a/test/util/one_off_handler_test.dart
+++ b/test/util/one_off_handler_test.dart
@@ -25,7 +25,7 @@ void main() {
   });
 
   test("passes a request to a handler only once", () async {
-    var path = handler.create(expectAsync((request) {
+    var path = handler.create(expectAsync1((request) {
       expect(request.method, equals("GET"));
       return new shelf.Response.ok("good job!");
     }));
@@ -40,17 +40,17 @@ void main() {
   });
 
   test("passes requests to the correct handlers", () async {
-    var path1 = handler.create(expectAsync((request) {
+    var path1 = handler.create(expectAsync1((request) {
       expect(request.method, equals("GET"));
       return new shelf.Response.ok("one");
     }));
 
-    var path2 = handler.create(expectAsync((request) {
+    var path2 = handler.create(expectAsync1((request) {
       expect(request.method, equals("GET"));
       return new shelf.Response.ok("two");
     }));
 
-    var path3 = handler.create(expectAsync((request) {
+    var path3 = handler.create(expectAsync1((request) {
       expect(request.method, equals("GET"));
       return new shelf.Response.ok("three");
     }));