Refactor the barback tests somewhat.

pkg/barback/test/transformer/mock.dart

Index: pkg/barback/test/transformer/mock.dart
diff --git a/pkg/barback/test/transformer/mock.dart b/pkg/barback/test/transformer/mock.dart
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+++ b/pkg/barback/test/transformer/mock.dart
@@ -0,0 +1,132 @@
+// Copyright (c) 2013, 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.
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
+
+/// The abstract base class for transformers used to test barback.
+///
+/// This adds the ability to pause and resume different components of the
+/// transformers, and to tell whether they're running, when they start running,
+/// and how many times they've run.
+///
+/// Transformers extending this should override [doIsPrimary] and [doApply]
+/// rather than [isPrimary] and [apply], and they should use [getInput] and
+/// [getPrimary] rather than [transform.getInput] and [transform.primaryInput].
+abstract class MockTransformer extends Transformer {
+  /// The number of times the transformer has been applied.
+  int get numRuns => _numRuns;
+  var _numRuns = 0;
+
+  /// The number of currently running transforms.
+  int _runningTransforms = 0;
+
+  // A completer for pausing the transformer before it finishes running [apply].
+  Completer _apply;
+
+  // Completers for pausing the transformer before it finishes running
+  // [isPrimary].
+  final _isPrimary = new Map<AssetId, Completer>();
+
+  // Completers for pausing the transformer before it finishes getting inputs
+  // the [Transform].
+  final _getInput = new Map<AssetId, Completer>();
+
+  /// A future that completes when this transformer begins running.
+  ///
+  /// Once this transformer finishes running, this is reset to a new completer,
+  /// so it can be used multiple times.
+  Future get started => _started.future;
+  var _started = new Completer();
+
+  /// `true` if any transforms are currently running.
+  bool get isRunning => _runningTransforms > 0;
+
+  /// Causes the transformer to pause after running [apply] but before the
+  /// returned Future completes.
+  ///
+  /// This can be resumed by calling [resumeApply].
+  void pauseApply() {
+    _apply = new Completer();
+  }
+
+  /// Resumes the transformer's [apply] call after [pauseApply] was called.
+  void resumeApply() {
+    _apply.complete();
+    _apply = null;
+  }
+
+  /// Causes the transformer to pause after running [isPrimary] on the asset
+  /// with the given [name], but before the returned Future completes.
+  ///
+  /// This can be resumed by calling [resumeIsPrimary].
+  void pauseIsPrimary(String name) {
+    _isPrimary[new AssetId.parse(name)] = new Completer();
+  }
+
+  /// Resumes the transformer's [isPrimary] call on the asset with the given
+  /// [name] after [pauseIsPrimary] was called.
+  void resumeIsPrimary(String name) {
+    _isPrimary.remove(new AssetId.parse(name)).complete();
+  }
+
+  /// Causes the transformer to pause while loading the input with the given
+  /// [name]. This can be the primary input or a secondary input.
+  ///
+  /// This can be resumed by calling [resumeGetInput].
+  void pauseGetInput(String name) {
+    _getInput[new AssetId.parse(name)] = new Completer();
+  }
+
+  /// Resumes the transformer's loading of the input with the given [name] after
+  /// [pauseGetInput] was called.
+  void resumeGetInput(String name) {
+    _getInput.remove(new AssetId.parse(name)).complete();
+  }
+
+  /// Like [Transform.getInput], but respects [pauseGetInput].
+  ///
+  /// This is intended for use by subclasses of [MockTransformer].
+  Future<Asset> getInput(Transform transform, AssetId id) {
+    return newFuture(() {
+      if (_getInput.containsKey(id)) return _getInput[id].future;
+    }).then((_) => transform.getInput(id));
+  }
+
+  /// Like [Transform.primaryInput], but respects [pauseGetInput].
+  ///
+  /// This is intended for use by subclasses of [MockTransformer].
+  Future<Asset> getPrimary(Transform transform) =>
+      getInput(transform, transform.primaryId);
+
+  Future<bool> isPrimary(Asset asset) {
+    return newFuture(() => doIsPrimary(asset)).then((result) {
+      return newFuture(() {
+        if (_isPrimary.containsKey(asset.id)) {
+          return _isPrimary[asset.id].future;
+        }
+      }).then((_) => result);
+    });
+  }
+
+  Future apply(Transform transform) {
+    _numRuns++;
+    if (_runningTransforms == 0) _started.complete();
+    _runningTransforms++;
+    return newFuture(() => doApply(transform)).then((_) {
+      if (_apply != null) return _apply.future;
+    }).whenComplete(() {
+      _runningTransforms--;
+      if (_runningTransforms == 0) _started = new Completer();
+    });
+  }
+
+  /// The wrapped version of [isPrimary] for subclasses to override.
+  Future<bool> doIsPrimary(Asset asset);
+
+  /// The wrapped version of [doApply] for subclasses to override.
+  Future doApply(Transform transform);
+}