Add preliminary packages file support to dependency_grapher.

pkg/front_end/lib/compiler_options.dart

Index: pkg/front_end/lib/compiler_options.dart
diff --git a/pkg/front_end/lib/compiler_options.dart b/pkg/front_end/lib/compiler_options.dart
index fdebf6768f0d0f7bbc612edbb7edadc64d046c16..5230f4e47fb31f7025e7d65351f99091280f44a6 100644
--- a/pkg/front_end/lib/compiler_options.dart
+++ b/pkg/front_end/lib/compiler_options.dart
@@ -36,6 +36,8 @@ class CompilerOptions {
   ///
   /// If `null`, the ".packages" file will be found via the standard
   /// package_config search algorithm.
+  ///
+  /// If the empty string, no packages file will be used.
   String packagesFilePath;
 
   /// Paths to the input summary files (excluding the SDK summary).  These files