Handle complicated 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 AsyncMarker, ProcedureKind;
 
 import '../combinator.dart' show Combinator;
 
@@ skipping 393 matching lines @@
     if (existing != null) {
       if (existing != member) {
         scope.local[name] = existing.combineAmbiguousImport(name, member, this);
       }
       // TODO(ahe): handle duplicated names.
     } else {
       scope.local[name] = member;
     }
   }
 
+  /// Returns true if the export scope was modified.
   bool addToExportScope(String name, Builder member) {
     if (name.startsWith("_")) return false;
     if (member is PrefixBuilder) return false;
     Builder existing = exports[name];
+    if (existing == member) return false;
     if (existing != null) {
-      // TODO(ahe): handle duplicated names.
-      return false;
+      exports[name] = buildAmbiguousBuilder(name, existing, member, -1);
     } else {
       exports[name] = member;
     }
     return true;
   }
 
   int resolveTypes(_) {
     int typeCount = types.length;
     for (T t in types) {
       t.resolveIn(scope);
@@ skipping 95 matching lines @@
   }
 
   /// Called to register [procedure] as a factory whose types are collected in
   /// [factoryDeclaration]. Later, once the class has been built, we can
   /// synthesize type variables on the factory matching the class'.
   void addFactoryDeclaration(
       ProcedureBuilder procedure, DeclarationBuilder<T> factoryDeclaration) {
     factoryDeclarations[procedure] = factoryDeclaration;
   }
 }