Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/builder/procedure_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.procedure_builder;
 
 // Note: we're deliberately using AsyncMarker and ProcedureKind from kernel
 // outside the kernel-specific builders. This is simpler than creating
 // additional enums.
 import 'package:kernel/ast.dart' show
@@ skipping 36 matching lines @@
   bool get isRegularMethod => identical(ProcedureKind.Method, kind);
 
   bool get isGetter => identical(ProcedureKind.Getter, kind);
 
   bool get isSetter => identical(ProcedureKind.Setter, kind);
 
   bool get isOperator => identical(ProcedureKind.Operator, kind);
 
   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.
   Scope computeFormalParameterScope(Scope parent) {
     if (formals == null) return parent;
     Map<String, Builder> local = <String, Builder>{};
     for (FormalParameterBuilder formal in formals) {
       if (!formal.hasThis) {
         local[formal.name] = formal;
       }
     }
     return new Scope(local, parent, isModifiable: false);
   }
 
   /// This scope doesn't correspond to any scope specified in the Dart
   /// Programming Language Specifiction, 4th ed. It's an unspecified extension
   /// to support generic methods.
   Scope computeTypeParameterScope(Scope parent) {
     if (typeVariables == null) return parent;
     Map<String, Builder> local = <String, Builder>{};
     for (TypeVariableBuilder variable in typeVariables) {
       local[variable.name] = variable;
     }
     return new Scope(local, parent, isModifiable: false);
   }
 }