Add packages accessor to FileLoader

Change-Id: I928549435593d0be1e86e74a234417f87898dbb5
diff --git a/file_loader/file_loader.h b/file_loader/file_loader.h
index c6a132f..3af4b6c 100644
--- a/file_loader/file_loader.h
+++ b/file_loader/file_loader.h
@@ -22,6 +22,11 @@
 
   bool LoadPackagesMap(const std::string& packages);
 
+  // The path to the `.packages` file the packages map was loaded from.
+  const std::string& packages() const {
+    return packages_;
+  }
+
   // Fully resolved file paths to dependencies. For example,
   // "package:foo/bar.dart" will be resolved to
   // "/path/to/package/foo/lib/bar.dart".