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..3c6e5be11638b49de4a4172db1efbe29b584add9 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') &&

[ahe, 2017/05/29 15:49:55]

I'm concerned about performance of this change. This loops over all classes and
performs string manipulation on URIs.

In addition, would prefer if this invariant was broken by dart2js specific code.

[Siggi Cherem (dart-lang), 2017/05/30 19:22:01]

Good point - I'll take a stab at making this overridable by dart2js instead.

[Siggi Cherem (dart-lang), 2017/05/30 21:16:58]

Sent you https://codereview.chromium.org/2907333003/ to address this.

             blackListedClasses.contains(supertype)) {
+          // These types are rarely extended in more than one platform library
+          // but it can be on different libraries depending on the target
+          // platform (e.g. dart:core for VM, dart:_interceptors for dart2js).
           cls.addCompileTimeError(
               cls.charOffset,
               "'${supertype.name}' is restricted and can't be extended or "