Fix all strong mode warnings in protoc-plugin

lib/const_generator.dart

 // Copyright (c) 2013, 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 protoc.const_generator;
 
 import "indenting_writer.dart";
 
 /// Writes JSON data as a Dart constant expression.
 /// Accepts null, bool, num, String, and maps and lists.
@@ skipping 74 matching lines @@
       out.print(", ");
     }
     writeJsonConst(out, item);
     if (vertical) {
       out.println(",");
     }
     first = false;
   }
 }
 
-void _writeMapItems(IndentingWriter out, Map<String, dynamic> val,
+void _writeMapItems(IndentingWriter out, Map<dynamic, dynamic> val,
     {bool vertical: false}) {
   bool first = true;
-  for (String key in val.keys) {
+  for (var key in val.keys) {
     if (!first && !vertical) out.print(", ");
     _writeString(out, key);
     out.print(": ");
     writeJsonConst(out, val[key]);
     if (vertical) {
       out.println(",");
     }
     first = false;
   }
 }