FE: let targets override whether imports to internal platform libraries (dart:_foo) are allowed.

pkg/compiler/lib/src/kernel/dart2js_target.dart

Index: pkg/compiler/lib/src/kernel/dart2js_target.dart
diff --git a/pkg/compiler/lib/src/kernel/dart2js_target.dart b/pkg/compiler/lib/src/kernel/dart2js_target.dart
index a0b9d618da8f0f6a68be0d5d23b0f348fa411529..a12fd0c9639e29d6c7d0c94f4d611e65be66a8a9 100644
--- a/pkg/compiler/lib/src/kernel/dart2js_target.dart
+++ b/pkg/compiler/lib/src/kernel/dart2js_target.dart
@@ -51,6 +51,11 @@ class Dart2jsTarget extends Target {
       uri.scheme == 'dart' &&
       (uri.path == 'core' || uri.path == '_interceptors');
 
+  @override
+  bool allowPlatformPrivateLibraryAccess(Uri importer, Uri imported) =>
+      super.allowPlatformPrivateLibraryAccess(importer, imported) ||
+      maybeEnableNative(importer);
+
   @override
   bool enableNative(Uri uri) => maybeEnableNative(uri);