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

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

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/analysis/session.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 1020 matching lines @@
     });
     _replaceReturnTypeWithFuture(body, typeProvider);
   }
 
   @override
   DartEditBuilderImpl createEditBuilder(int offset, int length) {
     return new DartEditBuilderImpl(this, offset, length);
   }
 
   @override
-  void finalize() {
-    CompilationUnitElement unitElement = unit.element;
-    LibraryElement libraryElement = unitElement.library;
-    CompilationUnitElement definingUnitElement =
-        libraryElement.definingCompilationUnit;
-    if (definingUnitElement == unitElement) {
-      _addLibraryImports(libraryElement, librariesToImport);
-    } else {
-      (changeBuilder as DartChangeBuilder).addFileEdit(
-          definingUnitElement.source.fullName, (DartFileEditBuilder builder) {
-        (builder as DartFileEditBuilderImpl)
-            ._addLibraryImports(libraryElement, librariesToImport);
-      });
+  Future<Null> finalize() async {
+    if (librariesToImport.isNotEmpty) {
+      CompilationUnitElement unitElement = unit.element;
+      LibraryElement libraryElement = unitElement.library;
+      CompilationUnitElement definingUnitElement =
+          libraryElement.definingCompilationUnit;
+      if (definingUnitElement == unitElement) {
+        _addLibraryImports(libraryElement, librariesToImport);
+      } else {
+        await (changeBuilder as DartChangeBuilder).addFileEdit(
+            definingUnitElement.source.fullName, (DartFileEditBuilder builder) {
+          (builder as DartFileEditBuilderImpl)
+              ._addLibraryImports(libraryElement, librariesToImport);
+        });
+      }
     }
   }
 
   @override
   void importLibraries(Iterable<Source> libraries) {
     librariesToImport.addAll(libraries);
   }
 
   @override
   void replaceTypeWithFuture(
@@ skipping 336 matching lines @@
   }
 }
 
 class _InsertionDescription {
   final int offset;
   final bool insertEmptyLineBefore;
   final bool insertEmptyLineAfter;
   _InsertionDescription(
       this.offset, this.insertEmptyLineBefore, this.insertEmptyLineAfter);
 }