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

pkg/front_end/lib/src/fasta/loader.dart

Index: pkg/front_end/lib/src/fasta/loader.dart
diff --git a/pkg/front_end/lib/src/fasta/loader.dart b/pkg/front_end/lib/src/fasta/loader.dart
index 743dcf6eaf3e20bf9026f5a580d4ddb88275eebc..5a1953bcf67becb0e7f69772f455d1b2f958b8f1 100644
--- a/pkg/front_end/lib/src/fasta/loader.dart
+++ b/pkg/front_end/lib/src/fasta/loader.dart
@@ -102,9 +102,8 @@ abstract class Loader<L> {
       return library;
     });
     if (accessor != null &&
-        uri.scheme == "dart" &&
-        uri.path.startsWith("_") &&
-        accessor.uri.scheme != "dart") {
+        !target.backendTarget
+            .allowPlatformPrivateLibraryAccess(accessor.uri, uri)) {
       accessor.addCompileTimeError(
           messagePlatformPrivateLibraryAccess, charOffset, accessor.fileUri);
     }