Fix NPE when a resolver provider is given

pkg/analyzer_cli/lib/src/driver.dart

Index: pkg/analyzer_cli/lib/src/driver.dart
diff --git a/pkg/analyzer_cli/lib/src/driver.dart b/pkg/analyzer_cli/lib/src/driver.dart
index 44cc1d1ff7374681b8612959ff34a6f7c1f5d5c0..b78389390303923106f8b3b262076a09083b2500 100644
--- a/pkg/analyzer_cli/lib/src/driver.dart
+++ b/pkg/analyzer_cli/lib/src/driver.dart
@@ -523,7 +523,7 @@ class Driver implements CommandLineStarter {
   _PackageInfo _findPackages(CommandLineOptions options) {
     if (packageResolverProvider != null) {
       // The resolver provider will do all the work later.
-      return null;
+      return new _PackageInfo(null, null);
     }
 
     Packages packages;