Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/builder/library_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/library_builder.dart b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
index 3ebfc671ed17aa4acb9dc9c54cdfdd41347daf34..f5b0a48f0191087cfd260e98fc0082ebb4e0d8ce 100644
--- a/pkg/front_end/lib/src/fasta/builder/library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/library_builder.dart
@@ -18,7 +18,6 @@ import '../loader.dart' show
 
 import 'builder.dart' show
     Builder,
-    InvalidTypeBuilder,
     TypeBuilder;
 
 import 'scope.dart' show
@@ -58,7 +57,7 @@ abstract class LibraryBuilder<T extends TypeBuilder, R> extends Builder {
 
   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;