Add Observatory to sky dart_controller

mojo/dart/embedder/embedder.gni

Index: mojo/dart/embedder/embedder.gni
diff --git a/mojo/dart/embedder/embedder.gni b/mojo/dart/embedder/embedder.gni
index b5d3de30e4d768d03112e1fa3fb3a176d1bd601f..dd9dcdfb6e942109643d606b19ddefaffecb6fcc 100644
--- a/mojo/dart/embedder/embedder.gni
+++ b/mojo/dart/embedder/embedder.gni
@@ -48,6 +48,7 @@ template("dart_embedder_package") {
 #   String output (name of output file)
 #   List inputs (list of input files to be included)
 #   String table_name (name of symbol for resource table)
+#   String root_prefix (base directory of resources)
 # Optional invoker inputs:
 #   String input_directory (directory of resources that are recursively added)
 #   List deps
@@ -71,6 +72,8 @@ template("dart_embedder_resources") {
 
     inputs = [ script ] + invoker.inputs
 
+    root_prefix = rebase_path(invoker.root_prefix)
+
     args = [
       "--output",
       rebase_path(output),
@@ -81,7 +84,7 @@ template("dart_embedder_resources") {
       "--table_name",
       invoker.table_name,
       "--root_prefix",
-      rebase_path("//mojo/dart/embedder/"),
+      root_prefix,
     ]
     if (defined(invoker.input_directory)) {
       args += [