Use package names as import prefixes when generating code

lib/enum_generator.dart

Index: lib/enum_generator.dart
diff --git a/lib/enum_generator.dart b/lib/enum_generator.dart
index b09d9b7182a3ec1fc8b702ebc9b6ce33ed2e117b..1f451f1eaeccb807438609aa1dad7f784ae534e1 100644
--- a/lib/enum_generator.dart
+++ b/lib/enum_generator.dart
@@ -11,6 +11,7 @@ class EnumAlias {
 }
 
 class EnumGenerator implements ProtobufContainer {
+  final ProtobufContainer _parent;
   final String classname;
   final String fqname;
   final List<EnumValueDescriptorProto> _canonicalValues =
@@ -21,7 +22,8 @@ class EnumGenerator implements ProtobufContainer {
       EnumDescriptorProto descriptor,
       ProtobufContainer parent,
       GenerationContext context)
-    : classname = (parent == null || parent is FileGenerator) ?
+    : _parent = parent,
+      classname = (parent == null || parent is FileGenerator) ?
           descriptor.name : '${parent.classname}_${descriptor.name}',
       fqname = (parent == null || parent.fqname == null) ? descriptor.name :
           (parent.fqname == '.' ?
@@ -38,6 +40,8 @@ class EnumGenerator implements ProtobufContainer {
     context.register(this);
   }
 
+  String get package => _parent.package;
+
   void generate(IndentingWriter out) {
     out.addBlock('class ${classname} extends ProtobufEnum {', '}\n', () {
       // -----------------------------------------------------------------