Set the correct mime types for assets served via pub serve.

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

Index: sdk/lib/_internal/pub/lib/src/barback/server.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/server.dart b/sdk/lib/_internal/pub/lib/src/barback/server.dart
index d285cfad3e978b42c3cf1a08422ee316f8ee9203..0ecd417d0c8d313a06637df0811eb0cfb4233465 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/server.dart
@@ -9,6 +9,7 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:barback/barback.dart';
+import 'package:mime/mime.dart';
 import 'package:path/path.dart' as path;
 import 'package:stack_trace/stack_trace.dart';
 
@@ -114,6 +115,7 @@ class BarbackServer {
       return validateStream(asset.read()).then((stream) {
         _resultsController.add(
             new BarbackServerResult._success(request.uri, id));
+        request.response.headers.add('content-type', lookupMimeType(id.path));
         // TODO(rnystrom): Set content-type based on asset type.
         return Chain.track(request.response.addStream(stream)).then((_) {
           // Log successful requests both so we can provide debugging