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

sdk/lib/_internal/pub/test/serve/utils.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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_tests;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
@@ skipping 151 matching lines @@
 }
 
 void endPubServe() {
   _pubServer.kill();
 }
 
 /// Schedules an HTTP request to the running pub server with [urlPath] and
 /// verifies that it responds with a body that matches [expectation].
 ///
 /// [expectation] may either be a [Matcher] or a string to match an exact body.
-void requestShouldSucceed(String urlPath, expectation) {
+/// [headers] may be either a [Matcher] or a map to match an exact headers map.
+void requestShouldSucceed(String urlPath, expectation, {headers}) {
   schedule(() {
     return http.get("http://127.0.0.1:$_port/$urlPath").then((response) {
-      expect(response.body, expectation);
+      if (expectation != null) expect(response.body, expectation);
+      if (headers != null) expect(response.headers, headers);
     });
   }, "request $urlPath");
 }
 
 /// Schedules an HTTP request to the running pub server with [urlPath] and
 /// verifies that it responds with a 404.
 void requestShould404(String urlPath) {
   schedule(() {
     return http.get("http://127.0.0.1:$_port/$urlPath").then((response) {
       expect(response.statusCode, equals(404));
@@ skipping 54 matching lines @@
 /// socket. It omitted, request is JSON encoded to a string first.
 void webSocketShouldReply(request, expectation, {bool encodeRequest: true}) {
   schedule(() => _ensureWebSocket().then((_) {
     if (encodeRequest) request = JSON.encode(request);
     _webSocket.add(request);
     return _webSocketBroadcastStream.first.then((value) {
       expect(JSON.decode(value), expectation);
     });
   }), "send $request to web socket and expect reply that $expectation");
 }