Add missing build dependencies

mojo/dart/embedder/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # embedder specific template rules.
 import("embedder.gni")
 
 source_set("dart_controller_no_snapshot") {
   sources = [
     "$target_gen_dir/dart_embedder_patch_resources.cc",
@@ skipping 222 matching lines @@
   ]
 }
 
 action("pub_build_observatory") {
   sources =
       rebase_path(observatory_sources_gypi.sources,
                   "",
                   "$root_gen_dir/observatory_copy/dart/runtime/observatory")
 
   deps = [
+    ":copy_observatory",
     ":copy_observatory_deps",
   ]
 
   script = "//dart/tools/observatory_tool.py"
 
   inputs = [
     script,
   ]
   inputs += sources
 
@@ skipping 152 matching lines @@
     "--vm_input_bin",
     rebase_path("$target_gen_dir/vm_isolate_snapshot.bin"),
     "--input_bin",
     rebase_path("$target_gen_dir/isolate_snapshot.bin"),
     "--input_cc",
     rebase_path("//mojo/dart/embedder/snapshot.cc.tmpl"),
     "--output",
     rebase_path(output),
   ]
 }