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

pkg/kernel/lib/target/targets.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 kernel.target.targets;
 
 import '../ast.dart';
 import '../class_hierarchy.dart';
 import '../core_types.dart';
 import '../transformations/treeshaker.dart' show ProgramRoot;
 import 'flutter.dart' show FlutterTarget;
@@ skipping 111 matching lines @@
       {void logger(String msg)});
 
   /// Whether a platform library may define a restricted type, such as `bool`,
   /// `int`, `double`, `num`, and `String`.
   ///
   /// By default only `dart:core` may define restricted types, but some target
   /// implementations override this.
   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,
   /// it means something else to each target, and it is enabled under different
   /// circumstances for each target implementation. For example, the VM target
   /// enables it everywhere because of existing support for "dart-ext:" native
   /// extensions, but targets like dart2js only enable it on the core libraries.
   bool enableNative(Uri uri) => false;
 
   /// There are two variants of the `native` language extension. The VM expects
@@ skipping 80 matching lines @@
       bool isField: false,
       bool isLocalVariable: false,
       bool isDynamic: false,
       bool isSuper: false,
       bool isStatic: false,
       bool isConstructor: false,
       bool isTopLevel: false}) {
     return new InvalidExpression();
   }
 }