Throw a StateError in case no model was registered for a given kind

lib/src/db/model_db_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.
 
 part of gcloud.db;
 
 /// An implementation of [ModelDB] based on model class annotations.
 ///
 /// The two constructors will scan loaded dart libraries for classes with a
 /// [Kind] annotation.
@@ skipping 43 matching lines @@
   ModelDBImpl.fromLibrary(Symbol librarySymbol) {
     _initialize([mirrors.currentMirrorSystem().findLibrary(librarySymbol)]);
   }
 
   /// Converts a [datastore.Key] to a [Key].
   Key fromDatastoreKey(datastore.Key datastoreKey) {
     var namespace = new Partition(datastoreKey.partition.namespace);
     Key key = namespace.emptyKey;
     for (var element in datastoreKey.elements) {
       var type = _type2ModelDesc[_kind2ModelDesc[element.kind]];
-      assert (type != null);
+      if (type == null) {
+        throw new StateError(
+            'Could not find a model associated with kind "${element.kind}". '
+            'Please ensure a model class was annotated with '
+            '`@Kind(name: "${element.kind}")`.');
+      }
       key = key.append(type, id: element.id);
     }
     return key;
   }
 
   /// Converts a [Key] to a [datastore.Key].
   datastore.Key toDatastoreKey(Key dbKey) {
     List<datastore.KeyElement> elements = [];
     var currentKey = dbKey;
     while (!currentKey.isEmpty) {
@@ skipping 463 matching lines @@
         enforceFieldExists: false);
     // If superclass can't encode field, we return value here (and assume
     // it's primitive)
     // NOTE: Implicit assumption:
     // If value != null then superclass will return != null.
     // TODO: Ensure [value] is primitive in this case.
     if (primitiveValue == null) primitiveValue = value;
     return primitiveValue;
   }
 }