Don't include BuiltinTypeBuilder(s) into additional exports.

pkg/front_end/lib/src/fasta/source/source_library_builder.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_library_builder;
 
 import 'package:kernel/ast.dart' show ProcedureKind, Reference;
 
 import '../../base/resolve_relative_uri.dart' show resolveRelativeUri;
 
 import '../../scanner/token.dart' show Token;
 
 import '../builder/builder.dart'
     show
         Builder,
+        BuiltinTypeBuilder,
         ClassBuilder,
         ConstructorReferenceBuilder,
         FormalParameterBuilder,
         FunctionTypeBuilder,
         LibraryBuilder,
         MemberBuilder,
         MetadataBuilder,
         NamedTypeBuilder,
         PrefixBuilder,
         ProcedureBuilder,
@@ skipping 489 matching lines @@
     }
     if (!explicitCoreImport) {
       loader.coreLibrary.exportScope.forEach((String name, Builder member) {
         addToScope(name, member, -1, true);
       });
     }
     exportScope.forEach((String name, Builder member) {
       if (member.parent != this) {
         additionalExports ??= <Reference>[];
         Builder parent = member.parent;
-        if (parent is LibraryBuilder) {
+        if (parent is LibraryBuilder && member is! BuiltinTypeBuilder) {
           additionalExports.add(member.target.reference);
         }
       }
     });
   }
 
   @override
   void addToScope(String name, Builder member, int charOffset, bool isImport) {
     Map<String, Builder> map =
         member.isSetter ? importScope.setters : importScope.local;
@@ skipping 139 matching lines @@
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 
   Scope toScope(Scope parent) {
     return new Scope(members, setters, parent, name, isModifiable: false);
   }
 }