Reduce strong mode errors and warnings

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

Index: pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart b/pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart
index b3948c9e98016ec12e539ef47d1663974edcdb83..a5a05bbbe5071229585c2719910cd8d2161c6abf 100644
--- a/pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/type_declaration_builder.dart
@@ -10,6 +10,9 @@ import 'builder.dart' show
     ModifierBuilder,
     TypeBuilder;
 
+import 'package:meta/meta.dart' show
+  virtual;
+
 abstract class TypeDeclarationBuilder<T extends TypeBuilder, R>
     extends ModifierBuilder {
   final List<MetadataBuilder> metadata;
@@ -20,7 +23,8 @@ abstract class TypeDeclarationBuilder<T extends TypeBuilder, R>
 
   final List<T> types;
 
-  Builder parent;
+  // TODO(sigmund): remove @virtual, should not be needed, see #28601

[ahe, 2017/02/02 07:28:30]

I'm patient. I can wait until issue 28601 is fixed.

I don't want to import random packages for no other reason than working around
bugs in the analyzer.

[Siggi Cherem (dart-lang), 2017/02/02 16:30:35]

I understand, note that it's a 1-file package with just a few top-level
constants that are used for annotations. Nothing else:
https://github.com/dart-lang/sdk/blob/master/pkg/meta/lib/meta.dart

It was a temporary way to test out annotations for the type checker without
having to add syntax to the language until it gets finalized.

+  @virtual Builder parent;
 
   TypeDeclarationBuilder(this.metadata, this.modifiers, this.name, this.types,
       this.parent);