Fix all strong mode warnings in protoc-plugin

lib/const_generator.dart

Index: lib/const_generator.dart
diff --git a/lib/const_generator.dart b/lib/const_generator.dart
index a0833070e343609f605f281371d621e605e26f64..11b9cede362bf37bdc5e573c6f602a9c936bd4e1 100644
--- a/lib/const_generator.dart
+++ b/lib/const_generator.dart
@@ -92,10 +92,10 @@ void _writeListItems(IndentingWriter out, List val, {bool vertical: 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(": ");