Make pub strong-mode clean.

lib/src/system_cache.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:io';
 
 import 'package:path/path.dart' as p;
 
 import 'io.dart';
 import 'io.dart' as io show createTempDir;
@@ skipping 83 matching lines @@
     }
 
     return new Package.load(
         id.name, source(id.source).getDirectory(id), sources);
   }
 
   /// Determines if the system cache contains the package identified by [id].
   bool contains(PackageId id) {
     var source = this.source(id.source);
 
-    if (source is! CachedSource) {
-      throw new ArgumentError("Package $id is not cacheable.");
-    }
-
-    return source.isInSystemCache(id);
+    if (source is CachedSource) return source.isInSystemCache(id);
+    throw new ArgumentError("Package $id is not cacheable.");
   }
 
   /// Create a new temporary directory within the system cache.
   ///
   /// The system cache maintains its own temporary directory that it uses to
   /// stage packages into while downloading. It uses this instead of the OS's
   /// system temp directory to ensure that it's on the same volume as the pub
   /// system cache so that it can move the directory from it.
   String createTempDir() {
     var temp = ensureDir(tempDir);
     return io.createTempDir(temp, 'dir');
   }
 
   /// Deletes the system cache's internal temp directory.
   void deleteTempDir() {
     log.fine('Clean up system cache temp directory $tempDir.');
     if (dirExists(tempDir)) deleteEntry(tempDir);
   }
 }