Reduce strong mode errors and warnings

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

Index: pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart b/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
index fb4409225ba97b14cbc08f8a4b50f9729a8d1222..f9e601c118511826656bf0abe708729379bcded6 100644
--- a/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
@@ -54,7 +54,7 @@ abstract class ProcedureBuilder<T extends TypeBuilder> extends MemberBuilder {
 
   bool get isFactory => identical(ProcedureKind.Factory, kind);
 
-  void set body(statement);
+  void set body(covariant statement);
 
   /// This is the formal parameter scope as specified in the Dart Programming
   /// Language Specifiction, 4th ed, section 9.2.