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

pkg/analyzer/lib/src/task/model.dart

Index: pkg/analyzer/lib/src/task/model.dart
diff --git a/pkg/analyzer/lib/src/task/model.dart b/pkg/analyzer/lib/src/task/model.dart
index 8eaa83896b18585eb46ec6161348dabaedc33ccf..8021f3ea7f8d23a166f5f6a62d315be16f7c0e05 100644
--- a/pkg/analyzer/lib/src/task/model.dart
+++ b/pkg/analyzer/lib/src/task/model.dart
@@ -126,13 +126,21 @@ class TaskDescriptorImpl implements TaskDescriptor {
   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,
@@ -143,5 +151,13 @@ class TaskDescriptorImpl implements TaskDescriptor {
   }
 
   @override
+  bool isAppropriateFor(AnalysisTarget target) {
+    if (_isAppropriateFor == null) {
+      return true;
+    }
+    return _isAppropriateFor(target);
+  }
+
+  @override
   String toString() => name;
 }