switch package to strong mode

lib/messages/messages.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.
 
 /// Defines messages templates and an adapter for TransformLogger to be able
 /// report error messages from transformers and refer to them in a consistent
 /// manner long term.
 library code_transformers.messages;
 
 // Note: this library purposely doesn't depend on dart:io, dart:html, or barback
@@ skipping 166 matching lines @@
           'url': span.end.sourceUrl.toString(),
           'offset': span.end.offset,
           'line': span.end.line,
           'column': span.end.column,
         },
         'text': span.text,
       };
     }
     return data;
   }
+
   String toString() => '${toJson()}';
 }
 
 /// A table of entries, that clusters error messages by id.
 class LogEntryTable {
   final Map<MessageId, List<BuildLogEntry>> entries;
 
   LogEntryTable() : entries = new LinkedHashMap();
 
   /// Creates a new [LogEntryTable] from an encoded value produced via [toJson].
   factory LogEntryTable.fromJson(Map json) {
     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;
   }
 
   /// Serializes this entire table as JSON.
   Map toJson() {
     var res = {};
     entries.forEach((key, value) {
       res['$key'] = value.map((e) => e.toJson()).toList();
     });
     return res;
   }
+
   String toString() => '${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, () => []);
       values.addAll(other.entries[key]);
     }
   }
 }