Fix a "pub run" bug.

lib/src/executable.dart

Index: lib/src/executable.dart
diff --git a/lib/src/executable.dart b/lib/src/executable.dart
index f692c026ecddec99dcd1767ca01d506080ddfc08..7e577d04256b5c2d9ef3d1856974182e557b5e16 100644
--- a/lib/src/executable.dart
+++ b/lib/src/executable.dart
@@ -110,6 +110,14 @@ Future<int> runExecutable(Entrypoint entrypoint, String package,
     return exit_codes.NO_INPUT;
   }
 
+  // If we're running an executable directly from the filesystem, make sure that
+  // it knows where to load the packages. If it's a dependency's executable, for
+  // example, it may not have the right packages directory itself.
+  if (executableUrl.scheme == 'file') {
+    // TODO(nweiz): use a .packages file once sdk#23369 is fixed.
+    vmArgs.add('--package-root=${p.toUri(entrypoint.packagesDir)}');
+  }
+
   vmArgs.add(executableUrl.toString());
   vmArgs.addAll(args);