Allow overriding core types in dart:_interceptors.

pkg/front_end/lib/src/fasta/source/source_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.source_loader;
 
 import 'dart:async' show Future;
 
 import 'dart:typed_data' show Uint8List;
 
@@ skipping 373 matching lines @@
     for (ClassBuilder cls in allClasses) {
       if (cls.library.loader != this) continue;
       Set<ClassBuilder> directSupertypes = new Set<ClassBuilder>();
       target.addDirectSupertype(cls, directSupertypes);
       for (ClassBuilder supertype in directSupertypes) {
         if (supertype is EnumBuilder) {
           cls.addCompileTimeError(
               cls.charOffset,
               "'${supertype.name}' is an enum and can't be extended or "
               "implemented.");
-        } else if (cls.library != coreLibrary &&
+        } else if (!cls.library.uri.isScheme('dart') &&
             blackListedClasses.contains(supertype)) {
+          // TODO(sigmund): we may want to use a stricter check here. Today
+          // these classes are extended in dart:core (for VM) and
+          // dart:_interceptors (for dart2js).

[Siggi Cherem (dart-lang), 2017/05/10 18:14:22]

not sure that it is worth saying in the TODO: I dont' do so to keep fasta
agnostic of the platform it is used for.

[scheglov, 2017/05/10 18:27:47]

FWIW, in analyzer we use similar check - we allow extending special classes
(bool, num, int, double, String) in any "system library", which mean "dart"
scheme. We have a comment about this, but not TODO, because WAI.

  bool _checkForExtendsOrImplementsDisallowedClass(
      TypeName typeName, ErrorCode errorCode) {
    if (typeName.isSynthetic) {
      return false;
    }
    // The SDK implementation may implement disallowed types. For example,
    // JSNumber in dart2js and _Smi in Dart VM both implement int.
    if (_currentLibrary.source.isInSystemLibrary) {
      return false;
    }

           cls.addCompileTimeError(
               cls.charOffset,
               "'${supertype.name}' is restricted and can't be extended or "
               "implemented.");
         }
       }
       TypeBuilder mixedInType = cls.mixedInType;
       if (mixedInType != null) {
         bool isClassBuilder = false;
         if (mixedInType is NamedTypeBuilder) {
@@ skipping 69 matching lines @@
   /// Performs the second phase of top level initializer inference, which is to
   /// visit fields and top level variables in topologically-sorted order and
   /// assign their types.
   void performInitializerInference() {
     typeInferenceEngine.finishTopLevel();
     ticker.logMs("Performed initializer inference");
   }
 
   List<Uri> getDependencies() => sourceBytes.keys.toList();
 }