switch package to strong mode

lib/messages/messages.dart

Index: lib/messages/messages.dart
diff --git a/lib/messages/messages.dart b/lib/messages/messages.dart
index 56f3c0e22315734071b69be714372996eb99e272..5268f4718304d8a4f3fb88fecc7e7338a37f85e6 100644
--- a/lib/messages/messages.dart
+++ b/lib/messages/messages.dart
@@ -184,6 +184,7 @@ class BuildLogEntry {
     }
     return data;
   }
+
   String toString() => '${toJson()}';
 }
 
@@ -198,8 +199,9 @@ class LogEntryTable {
     var res = new LogEntryTable();
     for (String key in json.keys) {
       var id = new MessageId.fromJson(key);
-      res.entries[id] =
-          json[key].map((v) => new BuildLogEntry.fromJson(v)).toList();
+      res.entries[id] = (json[key] as Iterable)
+          .map((v) => new BuildLogEntry.fromJson(v))
+          .toList();
     }
     return res;
   }
@@ -212,11 +214,13 @@ class LogEntryTable {
     });
     return res;
   }
+
   String toString() => '${toJson()}';
 
   void add(BuildLogEntry entry) {
     entries.putIfAbsent(entry.message.id, () => []).add(entry);
   }
+
   void addAll(LogEntryTable other) {
     for (var key in other.entries.keys) {
       var values = entries.putIfAbsent(key, () => []);