Add Observatory to sky dart_controller

mojo/dart/embedder/BUILD.gn

Index: mojo/dart/embedder/BUILD.gn
diff --git a/mojo/dart/embedder/BUILD.gn b/mojo/dart/embedder/BUILD.gn
index cf509aa57ccb507bdbbc38d27f878d9558eca16d..625242a5bfba9b35d36d4e239403565695df78e4 100644
--- a/mojo/dart/embedder/BUILD.gn
+++ b/mojo/dart/embedder/BUILD.gn
@@ -69,6 +69,7 @@ dart_embedder_resources("generate_dart_embedder_patch_resources_cc") {
     "//mojo/dart/embedder/io/socket_patch.dart",
     "//mojo/dart/embedder/io/server_socket_patch.dart",
   ]
+  root_prefix = "//mojo/dart/embedder/"
   output = "$target_gen_dir/dart_embedder_patch_resources.cc"
   table_name = "dart_embedder_patch"
 }
@@ -143,6 +144,7 @@ dart_embedder_resources("generate_dart_embedder_service_isolate_resources_cc") {
     "//mojo/dart/embedder/vmservice/resources.dart",
     "//mojo/dart/embedder/vmservice/server.dart",
   ]
+  root_prefix = "//mojo/dart/embedder/"
   input_directory = "$root_out_dir/observatory/deployed/web/"
   output = "$target_gen_dir/dart_embedder_service_isolate_resources.cc"
   table_name = "dart_embedder_service_isolate"