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

README.md

Index: README.md
diff --git a/README.md b/README.md
index dfa9b06ed52318a014741360ee6f21727702a1d9..f2570ebf2b564acd54caa50bf9c3a7ec4dc76730 100644
--- a/README.md
+++ b/README.md
@@ -343,7 +343,7 @@ void main() {
   test("Stream.fromIterable() emits the values in the iterable", () {
     var stream = new Stream.fromIterable([1, 2, 3]);
 
-    stream.listen(expectAsync((number) {
+    stream.listen(expectAsync1((number) {
       expect(number, inInclusiveRange(1, 3));
     }, count: 3));
   });