Nicer message when attempting to use Observatory from dart_bootstrap, dart_precompiled_runtime or a…

runtime/bin/vmservice/server.dart

 // Copyright (c) 2013, 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;
 
 final bool silentObservatory = const bool.fromEnvironment('SILENT_OBSERVATORY');
 
 void serverPrint(String s) {
   if (silentObservatory) {
@@ skipping 278 matching lines @@
       return;
     }
 
     if (path == WEBSOCKET_PATH) {
       WebSocketTransformer.upgrade(request).then((WebSocket webSocket) {
         new WebSocketClient(webSocket, _service);
       });
       return;
     }
 
+    if (assets == null) {
+      request.response.headers.contentType = ContentType.TEXT;
+      request.response.write("This VM was built without the Observatory UI.");
+      request.response.close();
+      return;
+    }
     Asset asset = assets[path];
     if (asset != null) {
       // Serving up a static asset (e.g. .css, .html, .png).
       request.response.headers.contentType = ContentType.parse(asset.mimeType);
       request.response.add(asset.data);
       request.response.close();
       return;
     }
     // HTTP based service request.
     try {
@@ skipping 100 matching lines @@
       _server = null;
       serverPrint('Could not shutdown Observatory HTTP server:\n$e\n$st\n');
       _notifyServerState("");
       onServerAddressChange(null);
       return this;
     });
   }
 }
 
 void _notifyServerState(String uri) native "VMServiceIO_NotifyServerState";