Make StreamIterator not delay pausing between requests.

tests/language/async_star_pause_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library async_star_pause_test;
+
+import "package:unittest/unittest.dart";
+import "dart:async";
+
+main() {
+  test("await for pauses stream during body", () async {
+    // Assumes await-for uses streamIterator.
+    var log = [];
+    var s = () async* {
+      for (int i = 0; i < 3; i++) {
+        log.add("$i-");
+        yield i;
+        // Should pause here until next iteration of await-for loop.
+        log.add("$i+");
+      }
+    }();
+    await for (var i in s) {
+      log.add("$i?");
+      await nextMicrotask();
+      log.add("$i!");
+    }
+    expect(log, ["0-", "0?", "0!", "0+",
+                 "1-", "1?", "1!", "1+",
+                 "2-", "2?", "2!", "2+"]);
+  });
+}
+Future nextMicrotask() => new Future.microtask((){});