Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/builder/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.library_builder;
 
 import '../combinator.dart' show
     Combinator;
 
 import '../errors.dart' show
     InputError;
 
 import '../export.dart' show
     Export;
 
 import '../loader.dart' show
     Loader;
 
 import 'builder.dart' show
     Builder,
-    InvalidTypeBuilder,
     TypeBuilder;
 
 import 'scope.dart' show
    Scope;
 
 abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
   final List<Export> exporters = <Export>[];
 
   final List<InputError> compileTimeErrors = <InputError>[];
 
@@ skipping 19 matching lines @@
   void addCompileTimeError(int charOffset, Object message) {
     InputError error = new InputError(uri, charOffset, message);
     compileTimeErrors.add(error);
     print(error.format());
   }
 
   bool addToExportScope(String name, Builder member);
 
   void addToScope(String name, Builder member);
 
-  InvalidTypeBuilder buildAmbiguousBuilder(
+  Builder buildAmbiguousBuilder(

[Siggi Cherem (dart-lang), 2017/02/02 04:26:58]

one of the subclasses returns a MixinAccessor, which is not an
InvalidTypeBuilder

       String name, Builder builder, Builder other);
 
   int finishStaticInvocations() => 0;
 }