Support configuring timeouts per-test and -group.

lib/src/frontend/timeout.dart

Index: lib/src/frontend/timeout.dart
diff --git a/lib/src/frontend/timeout.dart b/lib/src/frontend/timeout.dart
new file mode 100644
index 0000000000000000000000000000000000000000..d96407efd03256a3f89db9efbff3d8a25a27f17d
--- /dev/null
+++ b/lib/src/frontend/timeout.dart
@@ -0,0 +1,47 @@
+// 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.frontend.timeout;
+
+/// A class representing a modification to the default timeout for a test.
+///
+/// By default, a test will time out after 30 seconds. With [new Timeout], that
+/// can be overridden entirely; with [new Timeout.factor], it can be scaled
+/// relative to the default.
+class Timeout {
+  /// The timeout duration.
+  ///
+  /// If set, this overrides the default duration entirely. It will be
+  /// non-`null` only when [scaleFactor] is `null`.
+  final Duration duration;
+
+  /// The timeout factor.
+  ///
+  /// The default timeout will be multiplied by this to get the new timeout.
+  /// Thus a factor of 2 means that the test will take twice as long to time
+  /// out, and a factor of 0.5 means that it will time out twice as quickly.
+  final num scaleFactor;
+
+  /// Declares an absolute timeout that overrides the default.
+  const Timeout(this.duration)
+      : scaleFactor = null;
+
+  /// Declares a relative timeout that scales the default.
+  const Timeout.factor(this.scaleFactor)
+      : duration = null;
+
+  /// Returns a new [Timeout] that merges [this] with [other].
+  ///
+  /// If [other] declares a [duration], that takes precedence. Otherwise, this
+  /// timeout's [duration] or [factor] are multiplied by [other]'s [factor].
+  Timeout merge(Timeout other) {
+    if (other.duration != null) return new Timeout(other.duration);
+    if (duration != null) return new Timeout(duration * other.scaleFactor);
+    return new Timeout.factor(scaleFactor * other.scaleFactor);
+  }
+
+  /// Returns a new [Duration] from applying [this] to [base].
+  Duration apply(Duration base) =>
+      duration == null ? base * scaleFactor : duration;
+}