Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/builder/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.builder;
 
 import '../errors.dart' show
     internalError;
 
 export 'class_builder.dart' show
@@ skipping 71 matching lines @@
 abstract class Builder {
   /// Used when multiple things with the same name are declared within the same
   /// parent. Only used for declarations, not for scopes.
   ///
   // TODO(ahe): Move to member builder or something. Then we can make
   // this a const class.
   Builder next;
 
   /// Resolve types (lookup names in scope) recorded in this builder and return
   /// the number of types resolved.
-  int resolveTypes(Builder parent) => 0;
+  int resolveTypes(covariant Builder parent) => 0;
 
   /// Resolve constructors (lookup names in scope) recorded in this builder and
   /// return the number of constructors resolved.
-  int resolveConstructors(Builder parent) => 0;
+  int resolveConstructors(covariant Builder parent) => 0;
 
   /// Look for methods with the same name as their enclosing class and convert
   /// them to constructors. Return the number of methods converted to
   /// constructors.
-  int convertConstructors(Builder parent) => 0;
+  int convertConstructors(covariant Builder parent) => 0;
 
   /// This builder and [other] has been imported into [library] using [name].
   ///
   /// This method handles this case according to the Dart language
   /// specification.
   Builder combineAmbiguousImport(String name, Builder other,
       LibraryBuilder library) {
     if (other == this) return this;
     bool isLocal = false;
     Builder preferred;
@@ skipping 57 matching lines @@
 
   static Uri getUri(Builder builder) {
     if (builder == null) return internalError("Builder is null.");
     while (builder != null) {
       if (builder is LibraryBuilder) return builder.uri;
       builder = builder.parent;
     }
     return internalError("No library parent.");
   }
 }