Change IllegalArgumentException to ArgumentError.

utils/pub/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.
 
 #library('system_cache');
 
 #import('io.dart');
 #import('package.dart');
 #import('source.dart');
 #import('source_registry.dart');
@@ skipping 41 matching lines @@
 
   /**
    * Ensures that the package identified by [id] is installed to the cache,
    * loads it, and returns it.
    *
    * It is an error to try installing a package from a source with `shouldCache
    * == false` to the system cache.
    */
   Future<Package> install(PackageId id) {
     if (!id.source.shouldCache) {
-      throw new IllegalArgumentException("Package $id is not cacheable.");
+      throw new ArgumentError("Package $id is not cacheable.");
     }
 
     var pending = _pendingInstalls[id];
     if (pending != null) return pending;
 
     var future = id.source.installToSystemCache(id);
     always(future, () => _pendingInstalls.remove(id));
     _pendingInstalls[id] = future;
     return future;
   }
 }