Allow overriding core types in dart:_interceptors.

pkg/front_end/lib/src/fasta/source/source_loader.dart

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index 47a43b20de120c2f89392d5ff086435ef173d914..84106bf90d73914808926fe0836c653bbd131b6a 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -391,8 +391,11 @@ class SourceLoader<L> extends Loader<L> {
               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 "