- Avoid using a streamed receive port for single messages.

runtime/bin/vmservice/loader.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS 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.
 
 part of vmservice_io;
 
 var _httpClient;
 
-void _loadHttp(sendPort, uri) {
+// Send a response to the requesting isolate.
+void _sendResponse(SendPort sp, int id, dynamic data) {
+  assert((data is List<int>) || (data is String));
+  var msg = new List(2);
+  msg[0] = id;
+  msg[1] = data;
+  sp.send(msg);
+}
+
+void _loadHttp(SendPort sp, int id, Uri uri) {
   if (_httpClient == null) {
     _httpClient = new HttpClient()..maxConnectionsPerHost = 6;
   }
   _httpClient.getUrl(uri)
     .then((HttpClientRequest request) => request.close())
     .then((HttpClientResponse response) {
       var builder = new BytesBuilder(copy: false);
       response.listen(
           builder.add,
           onDone: () {
             if (response.statusCode != 200) {
-              var msg = 'Failure getting $uri: '
-                        '${response.statusCode} ${response.reasonPhrase}';
-              sendPort.send(msg);
+              var msg = "Failure getting $uri:\n"
+                        "  ${response.statusCode} ${response.reasonPhrase}";
+              _sendResponse(sp, id, msg);
             } else {
-              sendPort.send(builder.takeBytes());
+              _sendResponse(sp, id, builder.takeBytes());
             }
           },
           onError: (e) {
-            sendPort.send(e.toString());
+            _sendResponse(sp, d, e.toString());
           });
     })
     .catchError((e) {
-      sendPort.send(e.toString());
+      _sendResponse(sp, id, e.toString());
     });
   // It's just here to push an event on the event loop so that we invoke the
   // scheduled microtasks.
   Timer.run(() {});
 }
 
-void _loadFile(sendPort, path) {
+void _loadFile(SendPort sp, int id, Uri uri) {
+  var path = uri.toFilePath();
   var sourceFile = new File(path);
   sourceFile.readAsBytes().then((data) {
-    sendPort.send(data);
+    _sendResponse(sp, id, data);
   },
   onError: (e) {
-    sendPort.send(e.toString());
+    var err = "Error loading $uri:\n  $e";
+    _sendResponse(sp, id, err);
   });
 }
 
 var dataUriRegex = new RegExp(
     r"data:([\w-]+/[\w-]+)?(;charset=([\w-]+))?(;base64)?,(.*)");
 
-void _loadDataUri(sendPort, uri) {
+void _loadDataUri(SendPort sp, int id, Uri uri) {
   try {
     var match = dataUriRegex.firstMatch(uri.toString());
     if (match == null) throw "Malformed data uri";
 
     var mimeType = match.group(1);
     var encoding = match.group(3);
     var maybeBase64 = match.group(4);
     var encodedData = match.group(5);
 
     if (mimeType != "application/dart") {
       throw "MIME-type must be application/dart";
     }
     if (encoding != "utf-8") {
       // Default is ASCII. The C++ portion of the embedder assumes UTF-8.
       throw "Only utf-8 encoding is supported";
     }
     if (maybeBase64 != null) {
       throw "Only percent encoding is supported";
     }
 
     var data = UTF8.encode(Uri.decodeComponent(encodedData));
-    sendPort.send(data);
+    _sendResponse(sp, id, data);
   } catch (e) {
-    sendPort.send("Invalid data uri ($uri) $e");
+    _sendResponse(sp, id, "Invalid data uri ($uri):\n  $e");
   }
 }
 
 _processLoadRequest(request) {
-  var sp = request[0];
-  var uri = Uri.parse(request[1]);
+  SendPort sp = request[0];
+  int id = request[1];
+  String resource = request[2];
+  var uri = Uri.parse(request[2]);
   if (uri.scheme == 'file') {
-    _loadFile(sp, uri.toFilePath());
+    _loadFile(sp, id, uri);
   } else if ((uri.scheme == 'http') || (uri.scheme == 'https')) {
-    _loadHttp(sp, uri);
+    _loadHttp(sp, id, uri);
   } else if ((uri.scheme == 'data')) {
-    _loadDataUri(sp, uri);
+    _loadDataUri(sp, id, uri);
   } else {
     sp.send('Unknown scheme (${uri.scheme}) for $uri');
   }
 }