Fix issue with importing packages where the package name included a dot

lib/enum_generator.dart

Index: lib/enum_generator.dart
diff --git a/lib/enum_generator.dart b/lib/enum_generator.dart
index 1f451f1eaeccb807438609aa1dad7f784ae534e1..771dd12eff08fccc08908b03110c57ae37a6802b 100644
--- a/lib/enum_generator.dart
+++ b/lib/enum_generator.dart
@@ -10,7 +10,7 @@ class EnumAlias {
   EnumAlias(this.value, this.canonicalValue);
 }
 
-class EnumGenerator implements ProtobufContainer {
+class EnumGenerator extends ProtobufContainer {
   final ProtobufContainer _parent;
   final String classname;
   final String fqname;