Convert two tests to use async/await.

pkg/analysis_server/test/edit/assists_test.dart

 // Copyright (c) 2014, 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.
 
 library test.edit.assists;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/edit/edit_domain.dart';
 import 'package:analysis_server/src/plugin/server_plugin.dart';
@@ skipping 29 matching lines @@
   @override
   void setUp() {
     super.setUp();
     createProject();
     ExtensionManager manager = new ExtensionManager();
     ServerPlugin plugin = new ServerPlugin();
     manager.processPlugins([plugin]);
     handler = new EditDomainHandler(server, plugin);
   }
 
-  Future test_removeTypeAnnotation() {
+  Future test_removeTypeAnnotation() async {
     addTestFile('''
 main() {
   int v = 1;
 }
 ''');
-    return waitForTasksFinished().then((_) {
-      prepareAssists('v =');
-      _assertHasChange('Remove type annotation', '''
+    await waitForTasksFinished();
+    prepareAssists('v =');
+    _assertHasChange('Remove type annotation', '''
 main() {
   var v = 1;
 }
 ''');
-    });
   }
 
-  Future test_splitVariableDeclaration() {
+  Future test_splitVariableDeclaration() async {
     addTestFile('''
 main() {
   int v = 1;
 }
 ''');
-    return waitForTasksFinished().then((_) {
-      prepareAssists('v =');
-      _assertHasChange('Split variable declaration', '''
+    await waitForTasksFinished();
+    prepareAssists('v =');
+    _assertHasChange('Split variable declaration', '''
 main() {
   int v;
   v = 1;
 }
 ''');
-    });
   }
 
-  Future test_surroundWithIf() {
+  Future test_surroundWithIf() async {
     addTestFile('''
 main() {
   print(1);
   print(2);
 }
 ''');
-    return waitForTasksFinished().then((_) {
-      int offset = findOffset('  print(1)');
-      int length = findOffset('}') - offset;
-      prepareAssistsAt(offset, length);
-      _assertHasChange("Surround with 'if'", '''
+    await waitForTasksFinished();
+    int offset = findOffset('  print(1)');
+    int length = findOffset('}') - offset;
+    prepareAssistsAt(offset, length);
+    _assertHasChange("Surround with 'if'", '''
 main() {
   if (condition) {
     print(1);
     print(2);
   }
 }
 ''');
-    });
   }
 
   void _assertHasChange(String message, String expectedCode) {
     for (SourceChange change in changes) {
       if (change.message == message) {
         String resultCode =
             SourceEdit.applySequence(testCode, change.edits[0].edits);
         expect(resultCode, expectedCode);
         return;
       }
     }
     fail("Expected to find |$message| in\n" + changes.join('\n'));
   }
 }