Drop support for old SDK and shelf versions.

lib/src/runner/loader.dart

 // Copyright (c) 2015, 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.runner.loader;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
@@ skipping 134 matching lines @@
   /// [metadata] is the suite-level metadata for the test.
   Future<RunnerSuite> _loadVmFile(String path, Metadata metadata) async {
     var receivePort = new ReceivePort();
 
     var isolate;
     try {
       if (_config.pubServeUrl != null) {
         var url = _config.pubServeUrl.resolveUri(
             p.toUri(p.relative(path, from: 'test') + '.vm_test.dart'));
 
-        // TODO(nweiz): Remove new Future.sync() once issue 23498 has been fixed
-        // in two stable versions.
-        await new Future.sync(() async {
-          try {
-            isolate = await dart.spawnUri(url, {
-              'reply': receivePort.sendPort,
-              'metadata': metadata.serialize()
-            }, checked: true);
-          } on IsolateSpawnException catch (error) {
-            if (error.message.contains("OS Error: Connection refused") ||
-                error.message.contains("The remote computer refused")) {
-              throw new LoadException(path,
-                  "Error getting $url: Connection refused\n"
-                  'Make sure "pub serve" is running.');
-            } else if (error.message.contains("404 Not Found")) {
-              throw new LoadException(path,
-                  "Error getting $url: 404 Not Found\n"
-                  'Make sure "pub serve" is serving the test/ directory.');
-            }
+        try {
+          isolate = await Isolate.spawnUri(url, [], {
+            'reply': receivePort.sendPort,
+            'metadata': metadata.serialize()
+          }, checked: true);
+        } on IsolateSpawnException catch (error) {
+          if (error.message.contains("OS Error: Connection refused") ||
+              error.message.contains("The remote computer refused")) {
+            throw new LoadException(path,
+                "Error getting $url: Connection refused\n"
+                'Make sure "pub serve" is running.');
+          } else if (error.message.contains("404 Not Found")) {
+            throw new LoadException(path,
+                "Error getting $url: 404 Not Found\n"
+                'Make sure "pub serve" is serving the test/ directory.');
+          }
 
-            throw new LoadException(path, error);
-          }
-        });
+          throw new LoadException(path, error);
+        }
       } else {
         isolate = await dart.runInIsolate('''
 import "package:test/src/backend/metadata.dart";
 import "package:test/src/runner/vm/isolate_listener.dart";
 
 import "${p.toUri(p.absolute(path))}" as test;
 
 void main(_, Map message) {
   var sendPort = message['reply'];
   var metadata = new Metadata.deserialize(message['metadata']);
@@ skipping 54 matching lines @@
   Future close() {
     return _closeMemo.runOnce(() async {
       await Future.wait(_suites.map((suite) => suite.close()));
       _suites.clear();
 
       if (!_browserServerMemo.hasRun) return;
       await (await _browserServer).close();
     });
   }
 }