Make BrowserManager resilient to an early exit.

This also reverts the MultiChannel commits that were causing timeouts.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1062943005

lib/src/runner/browser/browser_manager.dart

@@ -50,7 +50,12 @@ class BrowserManager {
     // Create a nested MultiChannel because the iframe will be using a channel
     // wrapped within the host's channel.
     suiteChannel = new MultiChannel(suiteChannel.stream, suiteChannel.sink);
-    return suiteChannel.stream.first.then((response) {
+
+    // The stream may close before emitting a value if the browser is killed
+    // prematurely (e.g. via Control-C).
+    return maybeFirst(suiteChannel.stream).then((response) {
+      if (response == null) return null;
+
       if (response["type"] == "loadException") {
         return new Future.error(new LoadException(path, response["message"]));
       } else if (response["type"] == "error") {

lib/src/util/multi_channel.dart

@@ -93,9 +93,6 @@ class _MultiChannel extends StreamChannelMixin implements MultiChannel {
   /// The subscription to [_innerStream].
   StreamSubscription _innerStreamSubscription;
 
-  /// Whether this channel has been closed.
-  bool _closed = false;
-
   Stream get stream => _streamController.stream;
   final _streamController = new StreamController(sync: true);
 
@@ -201,9 +198,6 @@ class _MultiChannel extends StreamChannelMixin implements MultiChannel {
   /// Closes the virtual channel for which incoming messages have [inputId] and
   /// outgoing messages have [outputId].
   void _closeChannel(int inputId, int outputId) {
-    if (_closed) return;
-    _closed = inputId == 0;
-
     // A message without data indicates that the virtual channel has been
     // closed.
     _streamControllers.remove(inputId).close();

lib/src/utils.dart

@@ -184,6 +184,25 @@ Stream mergeStreams(Iterable<Stream> streamIter) {
   return controller.stream;
 }
 
+/// Returns the first value [stream] emits, or `null` if [stream] closes before
+/// emitting a value.
+Future maybeFirst(Stream stream) {
+  var completer = new Completer();
+
+  var subscription;
+  subscription = stream.listen((data) {
+    completer.complete(data);
+    subscription.cancel();
+  }, onError: (error, stackTrace) {
+    completer.completeError(error, stackTrace);
+    subscription.cancel();
+  }, onDone: () {
+    completer.complete();
+  });
+
+  return completer.future;
+}
+
 /// Returns a random base64 string containing [bytes] bytes of data.
 ///
 /// [seed] is passed to [math.Random]; [urlSafe] and [addLineSeparator] are