Improve parallelism when loading transformer plugins.

sdk/lib/_internal/pub/lib/src/barback/cycle_exception.dart

Index: sdk/lib/_internal/pub/lib/src/barback/cycle_exception.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/cycle_exception.dart b/sdk/lib/_internal/pub/lib/src/barback/cycle_exception.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c417ecf27ef526c238b6fd8a313941094ebfb9ab
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+++ b/sdk/lib/_internal/pub/lib/src/barback/cycle_exception.dart
@@ -0,0 +1,65 @@
+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 pub.barback.cycle_exception;
+
+import 'package:stack_trace/stack_trace.dart';
+
+import '../utils.dart';
+
+/// An exception thrown when a transformer dependency cycle is detected.
+///
+/// A cycle exception is usually produced within a deeply-nested series of
+/// calls. The API is designed to make it easy for each of these calls to add to
+/// the message so that the full reasoning for the cycle is made visible to the
+/// user.
+///
+/// Each call's individual message is called a "step". A [CycleException] is
+/// represented internally as a linked list of steps.
+class CycleException implements ApplicationException {
+  final innerError = null;
+  final Trace innerTrace = null;
+
+  /// The step for this exception.
+  final String _step;
+
+  /// The next exception in the linked list.
+  ///
+  /// [_next]'s steps come after [_step].
+  final CycleException _next;
+
+  /// A list of all steps in the cycle.
+  List<String> get steps {
+    if (_step == null) return [];
+
+    var exception = this;
+    var steps = [];
+    while (exception != null) {
+      steps.add(exception._step);
+      exception = exception._next;
+    }
+    return steps;
+  }
+
+  String get message {
+    var steps = this.steps;
+    if (steps.isEmpty) return "Transformer cycle detected.";
+    return "Transformer cycle detected:\n" +
+        steps.map((step) => "  $step").join("\n");
+  }
+
+  /// Creates a new [CycleException] with zero or one steps.
+  CycleException([this._step])
+      : _next = null;
+
+  CycleException._(this._step, this._next);
+
+  /// Returns a copy of [this] with [step] added to the beginning of [steps].
+  CycleException prependStep(String step) {
+    if (_step == null) return new CycleException(step);
+    return new CycleException._(step, this);
+  }
+
+  String toString() => message;
+}