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

pkg/compiler/lib/src/kernel/dart2js_target.dart

 // Copyright (c) 2017, 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 compiler.src.kernel.dart2js_target;
 
 import 'package:kernel/kernel.dart';
 import 'package:kernel/core_types.dart';
 import 'package:kernel/class_hierarchy.dart';
 import 'package:kernel/target/targets.dart';
@@ skipping 34 matching lines @@
         'dart:web_gl',
         'dart:web_sql',
       ];
 
   @override
   bool mayDefineRestrictedType(Uri uri) =>
       uri.scheme == 'dart' &&
       (uri.path == 'core' || uri.path == '_interceptors');
 
   @override
+  bool allowPlatformPrivateLibraryAccess(Uri importer, Uri imported) =>
+      super.allowPlatformPrivateLibraryAccess(importer, imported) ||
+      maybeEnableNative(importer);
+
+  @override
   bool enableNative(Uri uri) => maybeEnableNative(uri);
 
   @override
   bool get nativeExtensionExpectsString => false;
 
   @override
   void performModularTransformationsOnLibraries(
       CoreTypes coreTypes, ClassHierarchy hierarchy, List<Library> libraries,
       {void logger(String msg)}) {}
 
@@ skipping 18 matching lines @@
       bool isLocalVariable: false,
       bool isDynamic: false,
       bool isSuper: false,
       bool isStatic: false,
       bool isConstructor: false,
       bool isTopLevel: false}) {
     // TODO(sigmund): implement;
     return new InvalidExpression();
   }
 }