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

examples/dart/hello_world/hello/BUILD.gn

Index: examples/dart/hello_world/hello/BUILD.gn
diff --git a/examples/dart/hello_world/hello/BUILD.gn b/examples/dart/hello_world/hello/BUILD.gn
index 56cb2b260453a5c51d66ad4ddb5742eed22260ce..d1871f409c48dea8ecc3e3759284917fb3211407 100644
--- a/examples/dart/hello_world/hello/BUILD.gn
+++ b/examples/dart/hello_world/hello/BUILD.gn
@@ -4,7 +4,8 @@
 
 import("//mojo/public/dart/rules.gni")
 
-dartzip_packaged_application("hello") {
+dartzip_packaged_application("mojo_dart_hello_dartzip") {
+  output_name = "mojo_dart_hello"
   sources = [
     "main.dart",
   ]
@@ -12,3 +13,20 @@ dartzip_packaged_application("hello") {
     "//mojo/public/dart",
   ]
 }
+
+dart_pkg("mojo_dart_hello_pkg") {
+  entrypoints = [ "main.dart" ]
+  sources = [
+    "pubspec.yaml",
+  ]
+  deps = [
+    "//mojo/public/dart",
+  ]
+}
+
+group("hello") {
+  deps = [
+    ":mojo_dart_hello_dartzip",
+    ":mojo_dart_hello_pkg",
+  ]
+}