anchor.diff:            return new UnresolvedAccessor(this, n, token);
           }
           return new ThisPropertyAccessor(this, token, n, null, null);
  -      } else if (
  -          // Optimization, if [isPlatformLibrary] is false, this can't be
  -          // dart:_builtin.
  -          isPlatformLibrary &&
  -              name == "main" &&
  -              library.uri.path == "_builtin" &&
  -              member?.name == "_getMainClosure") {
  +      } else if (isBuiltinLibrary &&
  +          name == "main" &&
  +          member?.name == "_getMainClosure") {
           // TODO(ahe): https://github.com/dart-lang/sdk/issues/28989
           return new KernelNullLiteral()..fileOffset = offsetForToken(token);
         } else {
  
positive.code:       : enclosingScope = scope,
          library = library,
          enableNative = library.loader.target.enableNative(library),
          isPlatformLibrary = library.uri.scheme == 'dart',
          needsImplicitSuperInitializer =
              coreTypes.objectClass != classBuilder?.cls,
          typePromoter = _typeInferrer.typePromoter,
  
negative.code: 
    final bool enableNative;
  
    final bool isBuiltinLibrary;
  
    @override
    final Uri uri;
  