Add validation infrastructure for "pub lish".

utils/pub/entrypoint.dart

Index: utils/pub/entrypoint.dart
diff --git a/utils/pub/entrypoint.dart b/utils/pub/entrypoint.dart
index 521f2614c4bdae4b88cf55fbc88be08a0a56c703..d3af1fa53f92055e85d854aeed926ee7909ad1ed 100644
--- a/utils/pub/entrypoint.dart
+++ b/utils/pub/entrypoint.dart
@@ -51,6 +51,12 @@ class Entrypoint {
   Entrypoint(this.root, this.cache)
   : _installs = new Map<PackageId, Future<PackageId>>();
 
+  /// Loads the entrypoint from a package at [rootDir].
+  static Future<Entrypoint> load(String rootDir, SystemCache cache) {
+    return Package.load(null, rootDir, cache.sources).transform((package) =>
+        new Entrypoint(package, cache));
+  }
+
   /**
    * The path to this "packages" directory.
    */