Support loading transformer plugins from pub.

sdk/lib/_internal/pub/test/version_solver_test.dart

Index: sdk/lib/_internal/pub/test/version_solver_test.dart
diff --git a/sdk/lib/_internal/pub/test/version_solver_test.dart b/sdk/lib/_internal/pub/test/version_solver_test.dart
index d2b42b6be1750e4332124bb6a17bd3e6210f074b..bbe86acccdbc77e1c708a59f8114a85e599d5fde 100644
--- a/sdk/lib/_internal/pub/test/version_solver_test.dart
+++ b/sdk/lib/_internal/pub/test/version_solver_test.dart
@@ -1172,7 +1172,7 @@ Package mockPackage(String description, String version,
   var name = description.replaceFirst(new RegExp(r"-[^-]+$"), "");
   var pubspec = new Pubspec(
       name, new Version.parse(version), dependencies, devDependencies,
-      new PubspecEnvironment(sdkConstraint));
+      new PubspecEnvironment(sdkConstraint), []);
   return new Package.inMemory(pubspec);
 }