Add file watching to pub serve.

sdk/lib/_internal/pub/test/serve/utils.dart

Index: sdk/lib/_internal/pub/test/serve/utils.dart
diff --git a/sdk/lib/_internal/pub/test/serve/utils.dart b/sdk/lib/_internal/pub/test/serve/utils.dart
index 3a083923e61868e62a4dec541b797e4aea32cc01..0e11129aa783dae104acce27b1f7f50cb907313e 100644
--- a/sdk/lib/_internal/pub/test/serve/utils.dart
+++ b/sdk/lib/_internal/pub/test/serve/utils.dart
@@ -67,4 +67,38 @@ void requestShould404(String urlPath) {
       expect(response.statusCode, equals(404));
     });
   }, "request $urlPath");
+}
+
+/// Reads lines from pub serve's stdout until one equal to [output] is found.
+///
+/// The schedule will not proceed until the desired output is found. If not
+/// found, it will eventually time out.
+void waitForOutput(String output) {
+  findMatchingLine() {
+    return _pubServer.nextLine().then((line) {
+      if (line == output) return true;
+
+      // This line wasn't it, so ignore it and keep trying.
+      return findMatchingLine();
+    });
+  }
+
+  expect(findMatchingLine(), completion(isTrue));
+}
+
+/// Schedules pumping the event queue.
+void pumpEventQueue() {
+  schedule(() => _pumpEventQueue());
+}
+
+/// Returns a [Future] that completes after pumping the event queue [times]
+/// times. By default, this should pump the event queue enough times to allow
+/// any code to run, as long as it's not waiting on some external event.
+Future _pumpEventQueue([int times=20]) {
+  if (times == 0) return new Future.value();
+  // We use a delayed future to allow runAsync events to finish. The
+  // Future.value or Future() constructors use runAsync themselves and would
+  // therefore not wait for runAsync callbacks that are scheduled after invoking
+  // this method.
+  return new Future.delayed(Duration.ZERO, () => _pumpEventQueue(times - 1));
 }