Implement support for converting db values to datastore values with db.Query.filter

lib/src/datastore_impl.dart

 // Copyright (c) 2014, 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 gcloud.datastore_impl;
 
 import 'dart:async';
 
 import 'package:http/http.dart' as http;
 
@@ skipping 294 matching lines @@
     }
     pf.operator = operator;
     pf.property = new api.PropertyReference()..name = filter.name;
 
     // FIXME(Issue #5): Is this OK?
     var value = filter.value;
     if (filter.relation == datastore.FilterRelation.In) {
       if (value is List && value.length == 1) {
         value = value.first;
       } else {
-        throw new ArgumentError('List values not supported');
+        throw new ArgumentError('List values not supported (was: $value).');
       }
     }
 
     pf.value = _convertDatastore2ApiPropertyValue(value, true, lists: false);
     return new api.Filter()..propertyFilter = pf;
   }
 
   api.Filter _convertDatastoreAncestorKey2ApiFilter(datastore.Key key) {
     var pf = new api.PropertyFilter();
     pf.operator = 'HAS_ANCESTOR';
@@ skipping 375 matching lines @@
       return new Future.sync(() {
         throw new ArgumentError('Cannot call next() on last page.');
       });
     }
 
     return QueryPageImpl.runQuery(
         _api, _project, _nextRequest, _remainingNumberOfEntities)
         .catchError(DatastoreImpl._handleError);
   }
 }