Follow up fixes on handling of exports

pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart b/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
index e0baf8e7e9b70c0210eddcd9bd625a62639f6e1b..6c3771583938ea55141a08f3fef6c33b1c1f47de 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_library_builder.dart
@@ -147,18 +147,16 @@ class DillLibraryBuilder extends LibraryBuilder<KernelTypeBuilder, Library> {
   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);
           }