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

lib/file_generator.dart

Index: lib/file_generator.dart
diff --git a/lib/file_generator.dart b/lib/file_generator.dart
index cb908add7900f7463b5eaf7a619f42e0e05fde0a..9dc674e963de13321eb08887cf2a542e10fe4540 100644
--- a/lib/file_generator.dart
+++ b/lib/file_generator.dart
@@ -4,7 +4,7 @@
 
 part of protoc;
 
-class FileGenerator implements ProtobufContainer {
+class FileGenerator extends ProtobufContainer {
   final FileDescriptorProto _fileDescriptor;
   final ProtobufContainer _parent;
   final GenerationContext _context;
@@ -134,7 +134,7 @@ class FileGenerator implements ProtobufContainer {
       FileGenerator fileGenerator = _context.lookupFile(import);
       out.print("import '${_generatedFilePath(relativeProtoPath)}'");
       if (package != fileGenerator.package) {
-        out.print(' as ${fileGenerator.package}');
+        out.print(' as ${fileGenerator.packageImportPrefix}');
       }
       out.println(';');
     }