Fix performance regression for checking that retricted types are not

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 8ac15998ca36979a860781e2f94c099365076a21..dc290c1cca88e804511b3d7a9ded99b02ba8927f 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -336,6 +336,14 @@ class SourceLoader<L> extends Loader<L> {
     return output;
   }
 
+  /// Whether [library] is allowed to define classes that extend or implement
+  /// restricted types, such as `bool`, `int`, `double`, `num`, and `String`. By
+  /// default this is only allowed within the implementation of `dart:core`, but
+  /// some target implementations may need to override this to allow doing this
+  /// in other internal platform libraries.
+  bool canImplementRestrictedTypes(LibraryBuilder library) =>
+      library == coreLibrary;
+
   void checkSemantics() {
     List<ClassBuilder> allClasses = target.collectAllClasses();
     Iterable<ClassBuilder> candidates = cyclicCandidates(allClasses);
@@ -384,11 +392,8 @@ class SourceLoader<L> extends Loader<L> {
               cls.charOffset,
               "'${supertype.name}' is an enum and can't be extended or "
               "implemented.");
-        } else if (!cls.library.uri.isScheme('dart') &&
+        } else if (!canImplementRestrictedTypes(cls.library) &&
             blackListedClasses.contains(supertype)) {
-          // These types are rarely extended in more than one platform library
-          // but it can be on different libraries depending on the target
-          // platform (e.g. dart:core for VM, dart:_interceptors for dart2js).
           cls.addCompileTimeError(
               cls.charOffset,
               "'${supertype.name}' is restricted and can't be extended or "