Implement library access restrictions and privacy.

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.loader;
 
 import 'dart:async' show Future;
 
 import 'dart:collection' show Queue;
 
 import 'builder/builder.dart' show Builder, LibraryBuilder;
 
-import 'errors.dart' show InputError, firstSourceUri;
+import 'errors.dart' show InputError, firstSourceUri, printUnexpected;
 
 import 'target_implementation.dart' show TargetImplementation;
 
 import 'ticker.dart' show Ticker;
 
 abstract class Loader<L> {
   final Map<Uri, LibraryBuilder> builders = <Uri, LibraryBuilder>{};
 
   final Queue<LibraryBuilder> unparsedLibraries = new Queue<LibraryBuilder>();
 
@@ skipping 50 matching lines @@
       }
       if (uri.scheme == "dart") {
         target.readPatchFiles(library);
       }
       first ??= library;
       if (library.loader == this) {
         unparsedLibraries.addLast(library);
       }
       return library;
     });
-    // TODO(ahe): Check that [accessor] is allowed to access [builder].
+    if (accessor != null &&
+        uri.scheme == "dart" &&
+        uri.path.startsWith("_") &&
+        accessor.uri.scheme != "dart") {
+      const String message = "Can't access platform private library.";
+      printUnexpected(accessor.fileUri, charOffset, message);
+      errors.add(new InputError(accessor.fileUri, charOffset, message));
+    }
     return builder;
   }
 
   void ensureCoreLibrary() {
     if (coreLibrary == null) {
       read(Uri.parse("dart:core"), -1);
       assert(coreLibrary != null);
     }
   }
 
@@ skipping 67 matching lines @@
   Builder getFallThroughError() => target.getFallThroughError(this);
 
   Builder getNativeAnnotation() => target.getNativeAnnotation(this);
 
   Builder getNoSuchMethodError() => target.getNoSuchMethodError(this);
 }
 
 String format(double d, int fractionDigits, int width) {
   return d.toStringAsFixed(fractionDigits).padLeft(width);
 }