Add an Environment class that's exposed through RunnerSuite.

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 1e9ca1f2eaee9e5360466a3ed1cd70f63a793b99..2f4ff61b7a978770558fa92e422dba3f706013f2 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -14,7 +14,9 @@ import 'package:shelf/shelf.dart' as shelf;
 import 'package:stack_trace/stack_trace.dart';
 
 import 'backend/operating_system.dart';
+import 'util/cancelable_future.dart';
 import 'util/path_handler.dart';
+import 'util/stream_queue.dart';
 
 /// The maximum console line length.
 const _lineLength = 100;
@@ -321,6 +323,33 @@ Future maybeFirst(Stream stream) {
   return completer.future;
 }
 
+CancelableFuture cancelableNext(StreamQueue queue) {
+  var fork = queue.fork();
+  var completer = new CancelableCompleter(() => fork.cancel(immediate: true));
+  completer.complete(fork.next.then((_) {
+    fork.cancel();
+    return queue.next;
+  }));
+  return completer.future;
+}
+
+Future race(Iterable<CancelableFuture> futures) {
+  var completer = new Completer.sync();
+  for (var future in futures) {
+    future.then((value) {
+      if (!completer.isCompleted) completer.complete(value);
+    }).catchError((error, stackTrace) {
+      if (!completer.isCompleted) completer.completeError(error, stackTrace);
+    });
+  }
+
+  return completer.future.whenComplete(() {
+    for (var future in futures) {
+      future.cancel();
+    }
+  });
+}
+
 /// Returns a stream that emits [error] and [stackTrace], then closes.
 ///
 /// This is useful for adding errors to streams defined via `async*`.