Improve the error message when there is an infinite loop in tasks

pkg/analyzer/lib/src/task/driver.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 analyzer.src.task.driver;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/src/context/cache.dart';
@@ skipping 483 matching lines @@
    * the set of [WorkItem]s contained in the cycle (if there are overlapping
    * cycles, this is the set of all [WorkItem]s in the entire strongly
    * connected component).  Otherwise, `null`.
    */
   final List<WorkItem> dependencyCycle;
 
   /**
    * Initialize a newly created exception to represent a failed attempt to
    * perform the given [task] due to the given [dependencyCycle].
    */
-  InfiniteTaskLoopException(AnalysisTask task, this.dependencyCycle,
-      [this.cyclicPath])
-      : super(
-            'Infinite loop while performing task ${task.descriptor.name} for ${task.target}');
+  InfiniteTaskLoopException(AnalysisTask task, List<WorkItem> dependencyCycle,
+      [List<TargetedResult> cyclicPath])
+      : this.dependencyCycle = dependencyCycle,
+        this.cyclicPath = cyclicPath,
+        super(_composeMessage(task, dependencyCycle, cyclicPath));
+
+  /**
+   * Compose an error message based on the data we have available.
+   */
+  static String _composeMessage(AnalysisTask task,
+      List<WorkItem> dependencyCycle, List<TargetedResult> cyclicPath) {
+    StringBuffer buffer = new StringBuffer();
+    buffer.write('Infinite loop while performing task ');
+    buffer.write(task.descriptor.name);
+    buffer.write(' for ');
+    buffer.writeln(task.target);
+    buffer.writeln('  Dependency Cycle:');
+    for (WorkItem item in dependencyCycle) {
+      buffer.write('    ');
+      buffer.writeln(item);
+    }
+    if (cyclicPath != null) {
+      buffer.writeln('  Cyclic Path:');
+      for (TargetedResult result in cyclicPath) {
+        buffer.write('    ');
+        buffer.writeln(result);
+      }
+    }
+    return buffer.toString();
+  }
 }
 
 /**
  * Object used by [CycleAwareDependencyWalker] to report a single strongly
  * connected component of nodes.
  */
 class StronglyConnectedComponent<Node> {
   /**
    * The nodes contained in the strongly connected component.
    */
@@ skipping 313 matching lines @@
   final TaskManager taskManager;
 
   _WorkOrderDependencyWalker(this.taskManager, WorkItem startingNode)
       : super(startingNode);
 
   @override
   WorkItem getNextInput(WorkItem node, List<WorkItem> skipInputs) {
     return node.gatherInputs(taskManager, skipInputs);
   }
 }