First stab at a dev server in pub using barback.

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
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index 0000000000000000000000000000000000000000..bf6a4439e37cdd2bc8a4418ab6c2276e29f91150
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+++ b/sdk/lib/_internal/pub/lib/src/command/serve.dart
@@ -0,0 +1,159 @@
+// Copyright (c) 2013, 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 pub.command.serve;
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:barback/barback.dart';
+import 'package:path/path.dart' as path;
+
+import '../command.dart';
+import '../entrypoint.dart';
+import '../exit_codes.dart' as exit_codes;
+import '../log.dart' as log;
+import '../pub_package_provider.dart';
+import '../utils.dart';
+
+final _green = getPlatformString('\u001b[32m');
+final _red = getPlatformString('\u001b[31m');
+final _none = getPlatformString('\u001b[0m');
+
+/// Handles the `serve` pub command.
+class ServeCommand extends PubCommand {
+  String get description => "Run a local web development server.";
+  String get usage => 'pub serve';
+
+  ServeCommand() {
+    commandParser.addOption('port', defaultsTo: '8080',
+        help: 'The port to listen on.');
+  }
+
+  Future onRun() {
+    // The completer for the top-level future returned by the command. Only
+    // used to keep pub running (by not completing) and to pipe fatal errors
+    // to pub's top-level error-handling machinery.
+    var completer = new Completer();
+
+    return PubPackageProvider.create(entrypoint).then((provider) {
+      var port;
+      try {
+        port = int.parse(commandOptions['port']);
+      } on FormatException catch(_) {
+        log.error('Could not parse port "${commandOptions['port']}"');
+        this.printUsage();
+        exit(exit_codes.USAGE);
+      }
+
+      var barback = new Barback(provider);
+
+      barback.results.listen((result) {
+        if (result.succeeded) {
+          // TODO(rnystrom): Report using growl/inotify-send where available.
+          log.message("Build completed ${_green}successfully$_none");
+        } else {
+          log.message("Build completed with "
+              "${_red}${result.errors.length}$_none errors.");
+        }
+      });
+
+      barback.errors.listen((error) {
+        log.error("${_red}Build error:\n$error$_none");
+      });
+
+      // TODO(rnystrom): Watch file system and update sources again when they
+      // are added or modified.
+
+      HttpServer.bind("localhost", port).then((server) {
+        log.message("Serving ${entrypoint.root.name} "
+                    "on http://localhost:${server.port}");
+
+        // Add all of the visible files.
+        for (var package in provider.packages) {
+          barback.updateSources(provider.listAssets(package));
+        }
+
+        server.listen((request) {
+          var id = getIdFromUri(request.uri);
+          if (id == null) {
+            return notFound(request, "Path ${request.uri.path} is not valid.");
+          }
+
+          barback.getAssetById(id).then((asset) {
+            log.message(
+                "$_green${request.method}$_none ${request.uri} -> $asset");
+            // TODO(rnystrom): Set content-type based on asset type.
+            return request.response.addStream(asset.read()).then((_) {
+              request.response.close();
+            });

[nweiz, 2013/07/29 23:54:44]

If we're not doing the 500 thing for errors coming off this, at least add a TODO
to do so. If the filesystem goes briefly pear-shaped, we don't want that to
crash the server.

[Bob Nystrom, 2013/07/30 01:04:27]

Done.

+          }).catchError((error) {
+            log.error("$_red${request.method}$_none ${request.uri} -> $error");
+            if (error is! AssetNotFoundException) {
+              completer.completeError(error);
+              return;
+            }
+
+            notFound(request, error);
+          });
+        });
+      });
+
+      return completer.future;
+    });
+  }
+
+  /// Responds to [request] with a 404 response and closes it.
+  void notFound(HttpRequest request, message) {
+    request.response.statusCode = 404;
+    request.response.reasonPhrase = "Not Found";
+    request.response.write(message);
+    request.response.close();
+  }
+
+  AssetId getIdFromUri(Uri uri) {
+    var parts = path.url.split(uri.path);
+
+    // Strip the leading "/" from the URL.
+    parts.removeAt(0);
+
+    var isSpecial = false;
+
+    // Checks to see if [uri]'s path contains a special directory [name] that
+    // identifies an asset within some package. If so, maps the package name
+    // and path following that to be within [dir] inside that package.
+    AssetId _trySpecialUrl(String name, String dir) {
+      // Find the package name and the relative path in the package.
+      var index = parts.indexOf(name);
+      if (index == -1) return null;
+
+      // If we got here, the path *did* contain the special directory, which
+      // means we should not interpret it as a regular path, even if it's
+      // missing the package name after it, which makes it invalid here.
+      isSpecial = true;
+      if (index + 1 >= parts.length) return null;
+
+      var package = parts[index + 1];
+      var assetPath = path.url.join(dir,
+          path.url.joinAll(parts.skip(index + 2)));
+      return new AssetId(package, assetPath);
+    }
+
+    // See if it's "packages" URL.
+    var id = _trySpecialUrl("packages", "lib");
+    if (id != null) return id;
+
+    // See if it's an "assets" URL.
+    id = _trySpecialUrl("assets", "asset");
+    if (id != null) return id;
+
+    // If we got here, we had a path like "/packages" which is a special
+    // directory, but not a valid path since it lacks a following package name.
+    if (isSpecial) return null;
+
+    // Otherwise, it's a path in current package's web directory.
+    return new AssetId(entrypoint.root.name,
+        path.url.join("web", path.url.joinAll(parts)));
+  }
+}