Add dep on dart_snapshotter_bin label in dartx rule

mojo/public/dart/rules.gni

Index: mojo/public/dart/rules.gni
diff --git a/mojo/public/dart/rules.gni b/mojo/public/dart/rules.gni
index f02904109ed821f551f33aabbbcf58af3fa179fb..7ef8c57122b93231e59089891025b796d4c327a1 100644
--- a/mojo/public/dart/rules.gni
+++ b/mojo/public/dart/rules.gni
@@ -49,7 +49,9 @@ template("dartx") {
       rebase_path(snapshot),
     ]
 
-    deps = []
+    deps = [
+      dart_snapshotter_bin,

[jamesr, 2015/08/17 21:05:25]

If this variable isn't set, this will blow up at GN time.  There isn't a better
option at this point, however.

+    ]
     if (defined(invoker.deps)) {
       deps += invoker.deps
     }