Follow up fixes on handling of exports

pkg/front_end/lib/src/fasta/dill/dill_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.dill_library_builder;
 
 import 'dart:convert' show JSON;
 
 import 'package:kernel/ast.dart'
     show
@@ skipping 129 matching lines @@
   }
 
   @override
   String get fullNameForErrors {
     return library.name ?? "<library '${library.fileUri}'>";
   }
 
   void finalizeExports() {
     if (additionalExports != null) {
       for (List<String> additionalExport in additionalExports) {
-        Uri originUri = Uri.parse(additionalExport[0]);
+        String uriString = additionalExport[0];
         String name = additionalExport[1];
         Builder builder;
-        if (originUri == null) {
+        if (uriString == null) {
           builder = new KernelInvalidTypeBuilder(name, -1, null,
               templateUnspecified.withArguments(additionalExport[2]));
         } else {
+          Uri originUri = Uri.parse(uriString);
           DillLibraryBuilder library = loader.read(originUri, -1);
-          builder = library.exportScopeBuilder[name];
-          if (library != null) {
-            builder = library.exportScopeBuilder[name];
-          }
+          builder = library?.exportScopeBuilder[name];
           if (builder == null) {
             builder = new KernelInvalidTypeBuilder(name, -1, null);
           }
         }
         exportScopeBuilder.addMember(name, builder);
       }
     }
   }
 }