Make the FieldInfo.isDummy getter private (#272)

diff --git a/protobuf/lib/src/protobuf/builder_info.dart b/protobuf/lib/src/protobuf/builder_info.dart
index f8801b1..7de6062 100644
--- a/protobuf/lib/src/protobuf/builder_info.dart
+++ b/protobuf/lib/src/protobuf/builder_info.dart
@@ -63,7 +63,7 @@
     // Fields with tag number 0 are considered dummy fields added to avoid
     // index calculations add up. They should not be reflected in the following
     // maps.
-    if (!fi.isDummy) {
+    if (!fi._isDummy) {
       fieldInfo[fi.tagNumber] = fi;
       byTagAsString["${fi.tagNumber}"] = fi;
       byName[fi.name] = fi;
diff --git a/protobuf/lib/src/protobuf/field_info.dart b/protobuf/lib/src/protobuf/field_info.dart
index 974a419..70e9f8c 100644
--- a/protobuf/lib/src/protobuf/field_info.dart
+++ b/protobuf/lib/src/protobuf/field_info.dart
@@ -96,7 +96,7 @@
 
   /// Returns `true` if this represents a dummy field standing in for a field
   /// that has been removed by a program transformation.
-  bool get isDummy => tagNumber == 0;
+  bool get _isDummy => tagNumber == 0;
 
   bool get isRequired => _isRequired(type);
   bool get isRepeated => _isRepeated(type);