Make tests in resynthesize_common asynchronous.

pkg/analyzer/test/src/summary/resynthesize_ast_test.dart

Index: pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
diff --git a/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart b/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
index 94691e47093fac9ac41ef5e92248713b96cad5b8..d1bbb542d084ecbe48ca1a850aed8f7cbee9986e 100644
--- a/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
+++ b/pkg/analyzer/test/src/summary/resynthesize_ast_test.dart
@@ -4,6 +4,8 @@
 
 library analyzer.test.src.summary.resynthesize_ast_test;
 
+import 'dart:async';
+
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
@@ -250,8 +252,8 @@ abstract class _ResynthesizeAstTest extends ResynthesizeTest
   bool get shouldCompareLibraryElements;
 
   @override
-  LibraryElementImpl checkLibrary(String text,
-      {bool allowErrors: false, bool dumpSummaries: false}) {
+  Future<LibraryElementImpl> checkLibrary(String text,
+      {bool allowErrors: false, bool dumpSummaries: false}) async {
     Source source = addTestSource(text);
     LibraryElementImpl resynthesized = _encodeDecodeLibraryElement(source);
     LibraryElementImpl original = context.computeLibraryElement(source);