Make finalize() async and await adding imports, if any.

pkg/analyzer_plugin/lib/src/utilities/change_builder/change_builder_core.dart

Index: pkg/analyzer_plugin/lib/src/utilities/change_builder/change_builder_core.dart
diff --git a/pkg/analyzer_plugin/lib/src/utilities/change_builder/change_builder_core.dart b/pkg/analyzer_plugin/lib/src/utilities/change_builder/change_builder_core.dart
index e95657d4736f7fba8bbc0de1fec4719940657882..e996e68ea8a9ae48a40de75aaf2b5bfea661739b 100644
--- a/pkg/analyzer_plugin/lib/src/utilities/change_builder/change_builder_core.dart
+++ b/pkg/analyzer_plugin/lib/src/utilities/change_builder/change_builder_core.dart
@@ -50,7 +50,7 @@ class ChangeBuilderImpl implements ChangeBuilder {
     FileEditBuilderImpl builder = await createFileEditBuilder(path);
     buildFileEdit(builder);
     _change.addFileEdit(builder.fileEdit);
-    builder.finalize();
+    await builder.finalize();
   }
 
   /**
@@ -301,7 +301,7 @@ class FileEditBuilderImpl implements FileEditBuilder {
   /**
    * Finalize the source file edit that is being built.
    */
-  void finalize() {
+  Future<Null> finalize() async {
     // Nothing to do.
   }