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

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;
 
@@ skipping 84 matching lines @@
       if (uri.scheme == "dart") {
         target.readPatchFiles(library);
       }
       first ??= library;
       if (library.loader == this) {
         unparsedLibraries.addLast(library);
       }
       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);
     }
     return builder;
   }
 
   void ensureCoreLibrary() {
     if (coreLibrary == null) {
       read(Uri.parse("dart:core"), -1);
       assert(coreLibrary != null);
@@ skipping 73 matching lines @@
   Builder getDuplicatedFieldInitializerError() {
     return target.getDuplicatedFieldInitializerError(this);
   }
 
   Builder getNativeAnnotation() => target.getNativeAnnotation(this);
 }
 
 String format(double d, int fractionDigits, int width) {
   return d.toStringAsFixed(fractionDigits).padLeft(width);
 }