Make pub strong-mode clean.

lib/src/source.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.
 
 import 'dart:async';
 
 import 'package:pub_semver/pub_semver.dart';
 
 import 'package.dart';
 import 'pubspec.dart';
@@ skipping 42 matching lines @@
   /// The name of the source.
   ///
   /// Should be lower-case, suitable for use in a filename, and unique accross
   /// all sources.
   String get name;
 
   /// Whether this source can choose between multiple versions of the same
   /// package during version solving.
   ///
   /// Defaults to `false`.
-  final bool hasMultipleVersions = false;
+  bool get hasMultipleVersions => false;
 
   /// Records the system cache to which this source belongs.
   ///
   /// This should only be called once for each source, by
   /// [SystemCache.register]. It should not be overridden by base classes.
   BoundSource bind(SystemCache systemCache);
 
   /// Parses a [PackageRef] from a name and a user-provided [description].
   ///
   /// When a [Pubspec] is parsed, it reads in the description for each
@@ skipping 156 matching lines @@
   /// If the source is cached, this will be a path in the system cache.
   String getDirectory(PackageId id);
 
   /// Stores [pubspec] so it's returned when [describe] is called with [id].
   ///
   /// This is notionally protected; it should only be called by subclasses.
   void memoizePubspec(PackageId id, Pubspec pubspec) {
     _pubspecs[id] = pubspec;
   }
 }