Use more entities in program_builder, collector and registry

pkg/compiler/lib/src/elements/elements.dart

 // Copyright (c) 2012, 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 elements;
 
 import 'package:front_end/src/fasta/parser/async_modifier.dart'
     show AsyncModifier;
 
 import '../common.dart';
@@ skipping 725 matching lines @@
       CompilationUnitElement a, CompilationUnitElement b) {
     if (a == b) return 0;
     // Compilation units are compared only within the same library so we expect
     // the Uris to usually be clustered together with a common scheme and path
     // prefix.
     Uri aUri = a.script.readableUri;
     Uri bUri = b.script.readableUri;
     return '${aUri}'.compareTo('${bUri}');
   }
 
-  static List<Element> sortedByPosition(Iterable<Element> elements) {
+  static List<E> sortedByPosition<E extends Element>(Iterable<E> elements) {
     return elements.toList()..sort(compareByPosition);
   }
 
   static bool isFixedListConstructorCall(
       ConstructorEntity element, Send node, CommonElements commonElements) {
     return commonElements.isUnnamedListConstructor(element) &&
         node.isCall &&
         !node.arguments.isEmpty &&
         node.arguments.tail.isEmpty;
   }
@@ skipping 1277 matching lines @@
   /// by a field.
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }