Make a slightly better 404 page for pub serve.

sdk/lib/_internal/pub/lib/src/barback/base_server.dart

Index: sdk/lib/_internal/pub/lib/src/barback/base_server.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/base_server.dart b/sdk/lib/_internal/pub/lib/src/barback/base_server.dart
index df9fef3410cfa0d5190dc25cd4b97184e49df9ed..5cd96683d0c228b85abab76223c5e27b925c72e9 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/base_server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/base_server.dart
@@ -5,8 +5,10 @@
 library pub.barback.base_server;
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
+import 'package:barback/barback.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 import '../log.dart' as log;
@@ -65,17 +67,31 @@ abstract class BaseServer<T> {
   }
 
   /// Responds to [request] with a 404 response and closes it.
-  void notFound(HttpRequest request, message) {
-    logRequest(request, "404 Not Found");
+  ///
+  /// If [asset] is given, it is the ID of the asset that couldn't be found.
+  void notFound(HttpRequest request, {String error, AssetId asset}) {
+    logRequest(request, "Not Found");
 
     // Force a UTF-8 encoding so that error messages in non-English locales are
     // sent correctly.
     request.response.headers.contentType =
-        ContentType.parse("text/plain; charset=utf-8");
-
+        ContentType.parse("text/html; charset=utf-8");
     request.response.statusCode = 404;
     request.response.reasonPhrase = "Not Found";
-    request.response.write(message);
+
+    // TODO(rnystrom): Apply some styling to make it visually clear that this
+    // error is coming from pub serve itself.
+    request.response.writeln("<h1>404 Not Found</h1>");

[nweiz, 2014/04/01 19:20:28]

Now that this is HTML, it would be nice to provide at least a <title> tag.

[Bob Nystrom, 2014/04/01 19:40:57]

Done.

+    if (asset != null) {
+      request.response.writeln("<p>Could not find asset "
+          "<code>${HTML_ESCAPE.convert(asset.path)}</code> in package "
+          "<code>${HTML_ESCAPE.convert(asset.package)}</code>.</p>");
+    }
+
+    if (error != null) {
+      request.response.writeln("<p>Error: $error</p>");

[nweiz, 2014/04/01 19:20:28]

Don't forget to escape this too!

[Bob Nystrom, 2014/04/01 19:40:57]

Oops! Done.

+    }
+
     request.response.close();
   }