Add support for tasks to determine whether they are appropriate for a given target

pkg/analyzer/lib/src/task/model.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.model;
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/task/inputs.dart';
 import 'package:analyzer/task/model.dart';
 
@@ skipping 108 matching lines @@
   @override
   final CreateTaskInputs createTaskInputs;
 
   /**
    * A list of the analysis results that will be computed by the described task.
    */
   @override
   final List<ResultDescriptor> results;
 
   /**
+   * The function used to determine whether the described task is appropriate
+   * for a given target.
+   */
+  final IsAppropriateFor _isAppropriateFor;
+
+  /**
    * Initialize a newly created task descriptor to have the given [name] and to
    * describe a task that takes the inputs built using the given [createTaskInputs],
    * and produces the given [results]. The [buildTask] will be used to create
    * the instance of [AnalysisTask] thusly described.
    */
   TaskDescriptorImpl(
-      this.name, this.buildTask, this.createTaskInputs, this.results);
+      this.name, this.buildTask, this.createTaskInputs, this.results,
+      {IsAppropriateFor isAppropriateFor})

[Paul Berry, 2016/01/25 20:12:40]

Suggestion: make the default value of isAppropriateFor a static function that
returns true for any target.  Then the implementation of the isAppropriateFor()
method below can be simplified.

[Brian Wilkerson, 2016/01/25 20:27:36]

Good suggestion, but I modified it somewhat. Instead of using this as the
default value of the parameter I check for null in the assignment to the field.
That way, if a client passes in null explicitly we'll overwrite it.

+      : _isAppropriateFor = isAppropriateFor;
 
   @override
   AnalysisTask createTask(AnalysisContext context, AnalysisTarget target,
       Map<String, dynamic> inputs) {
     AnalysisTask task = buildTask(context, target);
     task.inputs = inputs;
     return task;
   }
 
   @override
+  bool isAppropriateFor(AnalysisTarget target) {
+    if (_isAppropriateFor == null) {
+      return true;
+    }
+    return _isAppropriateFor(target);
+  }
+
+  @override
   String toString() => name;
 }