Add file watching to pub serve.

sdk/lib/_internal/pub/lib/src/command/serve.dart

Index: sdk/lib/_internal/pub/lib/src/command/serve.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/serve.dart b/sdk/lib/_internal/pub/lib/src/command/serve.dart
index 69d92a1f6085e33b1ca9a41004e0b2438f2dfe4d..2e6d60ba4d47598548efdf35292c4382bb1e5ee9 100644
--- a/sdk/lib/_internal/pub/lib/src/command/serve.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/serve.dart
@@ -9,6 +9,7 @@ import 'dart:io';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as path;
+import 'package:watcher/watcher.dart';
 
 import '../command.dart';
 import '../entrypoint.dart';
@@ -81,6 +82,20 @@ class ServeCommand extends PubCommand {
         // Add all of the visible files.
         for (var package in provider.packages) {
           barback.updateSources(provider.listAssets(package));
+
+          // Watch the package for changes.
+          var packageDir = provider.getPackageDir(package);
+          var watcher = new DirectoryWatcher(packageDir);
+          watcher.events.listen((event) {

[nweiz, 2013/08/13 00:48:45]

If we're watching for any changes in the entire package, won't this start adding
things outside of lib/, asset/, and web/ to barback?

[Bob Nystrom, 2013/08/14 23:58:36]

Done.

+            var relativePath = path.relative(event.path, from: packageDir);
+            var id = new AssetId(package, relativePath);
+            log.message("${event.type} $id");

[nweiz, 2013/08/13 00:48:45]

I'm not a huge fan of exposing the weird "pkg|path" syntax to users. I'd rather
this be the physical path (for assets in the app package) or the
"package:"/"asset:" URI for foreign assets.

[Bob Nystrom, 2013/08/14 23:58:36]

Removed this. I was only using it as a kind of hack so that the tests could look
for that in the output. I have a cleaner way of writing those tests.

+            if (event.type == ChangeType.REMOVE) {
+              barback.removeSources([id]);
+            } else {
+              barback.updateSources([id]);
+            }
+          });
         }
 
         log.message("Serving ${entrypoint.root.name} "