Only load a certain number of test suites at once.

lib/src/runner/load_suite.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.load_suite;
 
 import 'dart:async';
 
 import 'package:stack_trace/stack_trace.dart';
 
@@ skipping 24 matching lines @@
   ///
   /// This will return `null` if the suite is unavailable for some reason (for
   /// example if an error occurred while loading it).
   final Future<Suite> suite;
 
   /// Creates a load suite named [name] on [platform].
   ///
   /// [body] may return either a [Suite] or a [Future] that completes to a
   /// [Suite]. Its return value is forwarded through [suite], although if it
   /// throws an error that will be forwarded through the suite's test.
+  ///
+  /// If the the load test is closed before [body] is complete, it will close
+  /// the suite returned by [body] once it completes.
   factory LoadSuite(String name, body(), {String platform}) {
     var completer = new Completer.sync();
     return new LoadSuite._(name, () {
       var invoker = Invoker.current;
       invoker.addOutstandingCallback();
 
       invoke(() async {
         try {
           var suite = await body();
-          if (completer.isCompleted) return;
+          if (completer.isCompleted) {
+            // If the load test has already been closed, close the suite it
+            // generated.
+            suite.close();
+            return;
+          }
+
           completer.complete(suite);
           invoker.removeOutstandingCallback();
         } catch (error, stackTrace) {
           registerException(error, stackTrace);
           if (!completer.isCompleted) completer.complete();
         }
       });
 
       // If the test is forcibly closed, exit immediately. It doesn't have any
       // cleanup to do that won't be handled by Loader.close.
@@ skipping 23 matching lines @@
         platform: suite.platform);
   }
 
   LoadSuite._(String name, void body(), this.suite, {String platform})
       : super([
         new LocalTest(name,
             new Metadata(timeout: new Timeout(new Duration(minutes: 5))),
             body)
       ], platform: platform);
 
+  /// A constructor used by [changeSuite].
+  LoadSuite._changeSuite(LoadSuite old, Future<Suite> this.suite)
+      : super(old.tests, platform: old.platform);
+
+  /// Creates a new [LoadSuite] that's identical to this one, but that
+  /// transforms [suite] once it's loaded.
+  ///
+  /// If [suite] completes to `null`, [change] won't be run.
+  Suite changeSuite(Suite change(Suite suite)) {
+    return new LoadSuite._changeSuite(this, suite.then((loadedSuite) {
+      if (loadedSuite == null) return null;
+      return change(loadedSuite);
+    }));
+  }
+
   /// Runs the test and returns the suite.
   ///
   /// Rather than emitting errors through a [LiveTest], this just pipes them
   /// through the return value.
   Future<Suite> getSuite() async {
     var liveTest = await tests.single.load(this);
     liveTest.onPrint.listen(print);
     await liveTest.run();
 
     if (liveTest.errors.isEmpty) return await suite;
 
     var error = liveTest.errors.first;
     await new Future.error(error.error, error.stackTrace);
     throw 'unreachable';
   }
 }