Fix analyzer errors/warnings for Pub.

utils/pub/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.
 
 library source;
 
 import 'io.dart';
 import 'package.dart';
 import 'pubspec.dart';
 import 'system_cache.dart';
@@ skipping 125 matching lines @@
   }
 
   /**
    * Returns the directory in the system cache that the package identified by
    * [id] should be installed to. This should return a path to a subdirectory of
    * [systemCacheRoot].
    *
    * This doesn't need to be implemented if [shouldCache] is false, or if
    * [installToSystemCache] is implemented.
    */
-  String systemCacheDirectory(PackageId id) =>
-    join(systemCacheRoot, packageName(id.description));
+  String systemCacheDirectory(PackageId id) {
+    throw 'Source.systemCacheDirectory must be implemented if shouldCache is '
+        'true and installToSystemCache is not implemented.';
+  }
 
   /**
    * When a [Pubspec] or [LockFile] is parsed, it reads in the description for
    * each dependency. It is up to the dependency's [Source] to determine how
    * that should be interpreted. This will be called during parsing to validate
    * that the given [description] is well-formed according to this source. It
    * should return if the description is valid, or throw a [FormatException] if
    * not.
    *
    * [fromLockFile] is true when the description comes from a [LockFile], to
@@ skipping 30 matching lines @@
    * to JSON and YAML. It must also be equal to [id] according to
    * [descriptionsEqual].
    *
    * By default, this just returns [id].
    */
   Future<PackageId> resolveId(PackageId id) => new Future.immediate(id);
 
   /// Returns the source's name.
   String toString() => name;
 }