Dart: Adds a program to create snapshots of Mojo apps.

examples/dart/hello_world/world/BUILD.gn

Index: examples/dart/hello_world/world/BUILD.gn
diff --git a/examples/dart/hello_world/world/BUILD.gn b/examples/dart/hello_world/world/BUILD.gn
index 25053ce37847a94cbd3646220e82e88d000d3a71..4b22e9acf7697ed077d4324ad53de77a05c8d8da 100644
--- a/examples/dart/hello_world/world/BUILD.gn
+++ b/examples/dart/hello_world/world/BUILD.gn
@@ -4,7 +4,8 @@
 
 import("//mojo/public/dart/rules.gni")
 
-dartzip_packaged_application("world") {
+dartzip_packaged_application("mojo_dart_world_dartzip") {
+  output_name = "mojo_dart_world"
   sources = [
     "main.dart",
   ]
@@ -20,3 +21,9 @@ dartzip_packaged_application("world") {
     strict = true
   }
 }
+
+group("world") {
+  deps = [
+    ":mojo_dart_world_dartzip",
+  ]
+}