First cut at _spawnUri rpc in the vm service.

sdk/lib/vmservice/message.dart

Index: sdk/lib/vmservice/message.dart
diff --git a/sdk/lib/vmservice/message.dart b/sdk/lib/vmservice/message.dart
index 08bb5b1a56343c156e1a39b10f84ef2ef76cbfa7..295214cb18716ab8d3ffbb8aade52e3e670579c8 100644
--- a/sdk/lib/vmservice/message.dart
+++ b/sdk/lib/vmservice/message.dart
@@ -130,6 +130,7 @@ class Message {
       case '_writeDevFSFile':
       case '_writeDevFSFiles':
       case '_readDevFSFile':
+      case '_spawnUri':
         return true;
       default:
         return false;
@@ -182,4 +183,4 @@ class Message {
 
 external bool sendIsolateServiceMessage(SendPort sp, List m);
 external void sendRootServiceMessage(List m);
-external void sendObjectRootServiceMessage(List m);
+external void sendObjectRootServiceMessage(List m);