Handle exports of invalid types.

pkg/front_end/lib/src/fasta/builder/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.builder;
 
 import '../errors.dart' show internalError;
 
 import '../messages.dart' show nit;
 
@@ skipping 40 matching lines @@
 export 'mixed_accessor.dart' show MixedAccessor;
 
 export 'scope.dart' show AccessErrorBuilder;
 
 export 'dynamic_type_builder.dart' show DynamicTypeBuilder;
 
 export 'function_type_builder.dart' show FunctionTypeBuilder;
 
 import 'library_builder.dart' show LibraryBuilder;
 
+import 'invalid_type_builder.dart' show InvalidTypeBuilder;
+
 abstract class Builder {
   /// Used when multiple things with the same name are declared within the same
   /// parent. Only used for declarations, not for scopes.
   ///
   // TODO(ahe): Move to member builder or something. Then we can make
   // this a const class.
   Builder next;
 
   /// The values of [parent], [charOffset], and [fileUri] aren't stored. We
   /// need to evaluate the memory impact of doing so, but want to ensure the
@@ skipping 17 matching lines @@
   /// return the number of constructors resolved.
   int resolveConstructors(covariant Builder parent) => 0;
 
   /// This builder and [other] has been imported into [library] using [name].
   ///
   /// This method handles this case according to the Dart language
   /// specification.
   Builder combineAmbiguousImport(
       String name, Builder other, LibraryBuilder library) {
     if (other == this) return this;
+    if (other is InvalidTypeBuilder) return other;
     bool isLocal = false;
     Builder preferred;
     Builder hidden;
     if (library.members[name] == this) {
       isLocal = true;
       preferred = this;
       hidden = other;
     } else if (getUri(other)?.scheme == "dart" &&
         getUri(this)?.scheme != "dart") {
       preferred = this;
@@ skipping 59 matching lines @@
 
   static Uri getUri(Builder builder) {
     if (builder == null) return internalError("Builder is null.");
     while (builder != null) {
       if (builder is LibraryBuilder) return builder.uri;
       builder = builder.parent;
     }
     return internalError("No library parent.");
   }
 }