Use UTF-8 when serving web pages for analysis server debug.

pkg/analysis_server/lib/src/server/http_server.dart

Index: pkg/analysis_server/lib/src/server/http_server.dart
diff --git a/pkg/analysis_server/lib/src/server/http_server.dart b/pkg/analysis_server/lib/src/server/http_server.dart
index da5ba355c855d3a9d7e11cc069721a5b649c4b9a..beda85cbeef23bc1314c2675b5663ebf3faa6040 100644
--- a/pkg/analysis_server/lib/src/server/http_server.dart
+++ b/pkg/analysis_server/lib/src/server/http_server.dart
@@ -118,7 +118,8 @@ class HttpAnalysisServer {
   void _returnUnknownRequest(HttpRequest request) {
     HttpResponse response = request.response;
     response.statusCode = HttpStatus.NOT_FOUND;
-    response.headers.add(HttpHeaders.CONTENT_TYPE, "text/plain");
+    response.headers.contentType =
+        new ContentType("text", "plain", charset: "utf-8");
     response.write('Not found');
     response.close();
   }