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

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

Index: sdk/lib/_internal/pub/test/serve/utils.dart
diff --git a/sdk/lib/_internal/pub/test/serve/utils.dart b/sdk/lib/_internal/pub/test/serve/utils.dart
index 039480307b309910d7877d216129c87859b8c2b6..6ae1e1ffb66644f018f076f1fd827fb0cd52a6b3 100644
--- a/sdk/lib/_internal/pub/test/serve/utils.dart
+++ b/sdk/lib/_internal/pub/test/serve/utils.dart
@@ -169,10 +169,12 @@ void endPubServe() {
 /// 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");
 }