Run "pub run" against the .packages file.

This reverts commit 7fbc104, which
reverted commit dc626af.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1272903002 .

lib/src/entrypoint.dart

@@ -76,6 +76,9 @@ class Entrypoint {
   /// The path to the entrypoint's "packages" directory.
   String get packagesDir => root.path('packages');
 
+  /// The path to the entrypoint's ".packages" file.
+  String get packagesFile => root.path('.packages');
+
   /// `true` if the entrypoint package currently has a lock file.
   bool get lockFileExists => _lockFile != null || entryExists(lockFilePath);
 

lib/src/executable.dart

@@ -120,7 +120,7 @@ Future<int> runExecutable(Entrypoint entrypoint, String package,
     // helpful for the subprocess to be able to spawn Dart with
     // Platform.executableArguments and have that work regardless of the working
     // directory.
-    vmArgs.add('--package-root=${p.toUri(p.absolute(entrypoint.packagesDir))}');
+    vmArgs.add('--packages=${p.toUri(p.absolute(entrypoint.packagesFile))}');
   }
 
   vmArgs.add(executableUrl.toString());