Add support for additional libraries to FE API

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 921758ac57377659a4b6c3fcd097856ea931294b..8653baa02915515c847126e816f5d9e052d1ec23 100644
--- a/pkg/front_end/lib/compiler_options.dart
+++ b/pkg/front_end/lib/compiler_options.dart
@@ -127,4 +127,10 @@ class CompilerOptions {
   /// either absolute or relative URIs. Absolute URIs are read directly, while
   /// relative URIs are resolved from the [sdkRoot].
   Map<Uri, List<Uri>> targetPatches = {};
+
+  /// Additional core libraries to be loaded when building a program.
+  // TODO(sigmund): delete. Ideally building a program only needs what's
+  // reachable and we can use kernelForBuildUnit when creating a snapshot of the
+  // SDK itself.
+  List<Uri> additionalLibraries = [];
 }