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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+++ b/sdk/lib/_internal/pub/lib/src/command/serve.dart
@@ -0,0 +1,142 @@
+// 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() {
+    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) {
+          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");
+      });
+
+      // Add all of the visible files.
+      for (var package in provider.packages) {
+        barback.updateSources(provider.listAssets(package));
+      }
+      // TODO(rnystrom): Watch file system and update sources again when they
+      // are added or modified.
+
+      HttpServer.bind(InternetAddress.LOOPBACK_IP_V4, port).then((server) {
+        log.message("Serving ${entrypoint.root.name} "
+                    "on http://localhost:${server.port}");
+
+        server.listen((request) {
+          var id = getIdFromUri(request.uri);
+          barback.getAssetById(id).then((asset) {
+            log.message(
+                "$_green${request.method}$_none ${request.uri} -> $asset");
+            // TODO(rnystrom): Set content-type based on asset type.
+            return request.response.addStream(asset.read()).then((_) {
+              request.response.close();
+            });
+          }).catchError((error) {
+            log.error("$_red${request.method}$_none ${request.uri} -> $error");
+            if (error is AssetNotFoundException) {
+              request.response.statusCode = 404;
+              request.response.reasonPhrase = "Not Found";
+              request.response.write(error);
+            } else {
+              request.response.statusCode = 500;
+              request.response.reasonPhrase = "Internal Server Error";
+              request.response.writeln(error);
+              request.response.writeln(getAttachedStackTrace(error));
+            }

[nweiz, 2013/07/29 20:23:15]

Error responses should have the content-type set to text/plain.

+            request.response.close();
+          });
+        });
+      });
+
+      // TODO(rnystrom): Hack! Return a future that never completes to leave
+      // pub running.
+      return new Completer().future;
+    });
+  }
+
+  AssetId getIdFromUri(Uri uri) {
+    var uriBuilder = new path.Builder(style: path.Style.url);

[nweiz, 2013/07/29 20:23:15]

I think the pattern we usually use for this is a top-level final private
variable. E.g.

    final _path = new path.Builder(style: path.Style.url);

[Bob Nystrom, 2013/07/29 21:45:43]

Added default builders for each style to path and changed this to use path.url.

+    var parts = uriBuilder.split(uri.path);
+
+    // Strip the leading "/" from the URL.
+    parts.removeAt(0);
+
+    // Special case: if the URL is "/", map it to "web/index.html".
+    // TODO(rnystrom): What about directory URLs? Should "foo" or "foo/" map
+    // to "foo/index.html"?
+    if (parts.isEmpty) parts = ["index.html"];
+
+    // 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 (index + 1 >= parts.length) return null;
+
+      var package = parts[index + 1];
+      var assetPath = uriBuilder.join(dir,
+          uriBuilder.joinAll(parts.skip(index + 2)));
+      return new AssetId(package, assetPath);
+    }

[nweiz, 2013/07/29 20:23:15]

Currently this won't 404 for "/packages" if there's a file named "packages" in
the web directory. You can just add a TODO for that.

[Bob Nystrom, 2013/07/29 21:45:43]

Made it do the right thing.

+
+    // 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;
+
+    // Otherwise, it's a path in current package's web directory.
+    return new AssetId(entrypoint.root.name,
+        uriBuilder.join("web", uriBuilder.joinAll(parts)));
+  }
+}