Handle missing files more gracefully in pub serve.

sdk/lib/_internal/pub/lib/src/utils.dart

Index: sdk/lib/_internal/pub/lib/src/utils.dart
diff --git a/sdk/lib/_internal/pub/lib/src/utils.dart b/sdk/lib/_internal/pub/lib/src/utils.dart
index 6a048cf1612a5184c6c0b0d309dc4cec16c561c9..aae5bd14fed7dfdf0e43b372699127c8d5294b20 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -168,6 +168,39 @@ void chainToCompleter(Future future, Completer completer) {
       onError: (e) => completer.completeError(e));
 }
 
+/// Ensures that [stream] can emit at least one value successfully (or close
+/// without any values).
+///
+/// For example, reading asynchronously from a non-existent file will return a
+/// stream that fails on the first chunk. In order to handle that more
+/// gracefully, you may want to check that the stream looks like it's working
+/// before you pipe the stream to something else.
+///
+/// This lets you do that. It returns a [Future] that completes to a [Stream]
+/// emitting the same values and errors as [stream], but only if at least one
+/// value can be read successfully. If an error occurs before any values are
+/// emitted, the returned Future completes to that error.
+Future<Stream> validateStream(Stream stream) {
+  var completer = new Completer<Stream>();
+  var controller = new StreamController();

[nweiz, 2013/08/12 20:26:31]

Make this sync? Since [stream] will already be async, adding an extra event loop
hop is probably unnecessary.

[Bob Nystrom, 2013/08/12 21:45:00]

Good call. Done.

+
+  stream.listen((value) {
+    // We got a value, so the stream is valid.
+    if (!completer.isCompleted) completer.complete(controller.stream);
+    controller.add(value);
+  }, onError: (error) {
+    // We got an error before any values, so the stream is invalid.
+    if (!completer.isCompleted) completer.completeError(error);

[nweiz, 2013/08/12 20:26:31]

If we get an error first, we should cancel our subscription to the stream.

[Bob Nystrom, 2013/08/12 21:45:00]

Done.

+    controller.addError(error);
+  }, onDone: () {
+    // It closed with no errors, so the stream is valid.
+    if (!completer.isCompleted) completer.complete(controller.stream);
+    controller.close();
+  });
+
+  return completer.future;
+}
+
 // TODO(nweiz): remove this when issue 7964 is fixed.
 /// Returns a [Future] that will complete to the first element of [stream].
 /// Unlike [Stream.first], this is safe to use with single-subscription streams.