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

pkg/kernel/lib/target/targets.dart

Index: pkg/kernel/lib/target/targets.dart
diff --git a/pkg/kernel/lib/target/targets.dart b/pkg/kernel/lib/target/targets.dart
index 3227f012c7b72a3ae7ef275653378886a0a37049..91671416072c3d03e01fbef3d7a1d8e2006d604a 100644
--- a/pkg/kernel/lib/target/targets.dart
+++ b/pkg/kernel/lib/target/targets.dart
@@ -129,6 +129,15 @@ abstract class Target {
   bool mayDefineRestrictedType(Uri uri) =>
       uri.scheme == 'dart' && uri.path == 'core';
 
+  /// Whether a library is allowed to import a platform private library.
+  ///
+  /// By default only `dart:*` libraries are allowed. May be overriden for
+  /// testing purposes.
+  bool allowPlatformPrivateLibraryAccess(Uri importer, Uri imported) =>
+      imported.scheme != "dart" ||
+      !imported.path.startsWith("_") ||
+      importer.scheme == "dart";
+
   /// Whether the `native` language extension is supported within [library].
   ///
   /// The `native` language extension is not part of the language specification,