Add "load from .dill" file capability and run a white-box test.

pkg/compiler/lib/src/commandline_options.dart

Index: pkg/compiler/lib/src/commandline_options.dart
diff --git a/pkg/compiler/lib/src/commandline_options.dart b/pkg/compiler/lib/src/commandline_options.dart
index 970cc64e985c1aab5f5d3273c12576e1c4b84384..c3195f0b6d9f3139738aa1d1f24fb48af3ccd876 100644
--- a/pkg/compiler/lib/src/commandline_options.dart
+++ b/pkg/compiler/lib/src/commandline_options.dart
@@ -30,6 +30,9 @@ class Flags {
   // Temporary flag to also integrate Kernel into global type inference.
   // TODO(efortuna): Remove when fully implemented and simply use "useKernel".
   static const String kernelGlobalInference = '--kernel-global-inference';
+  // Read input from a .dill file rather than a .dart input (use only in
+  // conjunction with --use-kernel=true).
+  static const String loadFromDill = '--read-dill';
   static const String minify = '--minify';
   static const String noFrequencyBasedMinification =
       '--no-frequency-based-minification';