Add simple Dart tasks

pkg/analyzer/lib/src/task/general.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.general;
 
 import 'package:analyzer/src/generated/engine.dart' hide AnalysisTask;
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/task/general.dart';
 import 'package:analyzer/task/model.dart';
 
 /**
  * A task that gets the contents of the source associated with an analysis
  * target.
  */
-class GetContentTask extends AnalysisTask {
+class GetContentTask extends SourceBasedAnalysisTask {
   /**
    * The task descriptor describing this kind of task.
    */
   static final TaskDescriptor DESCRIPTOR = new TaskDescriptor(
       'GET_CONTENT',
       createTask,
       buildInputs,
       <ResultDescriptor>[CONTENT, MODIFICATION_TIME]);
 
   /**
    * Initialize a newly created task to access the content of the source
    * associated with the given [target] in the given [context].
    */
   GetContentTask(InternalAnalysisContext context, AnalysisTarget target)
       : super(context, target);
 
   @override
-  String get description {
-    Source source = target.source;
-    if (source == null) {
-      return "get contents of <unknown source>";
-    }
-    return "get contents of ${source.fullName}";
-  }
-
-  @override
   TaskDescriptor get descriptor => DESCRIPTOR;
 
   @override
   internalPerform() {
     Source source = getRequiredSource();
 
     TimestampedData<String> data = context.getContents(source);
     outputs[CONTENT] = data.data;
     outputs[MODIFICATION_TIME] = data.modificationTime;
   }
 
   /**
    * Return a map from the names of the inputs of this kind of task to the task
    * input descriptors describing those inputs for a task with the given [target].
    */
   static Map<String, TaskInput> buildInputs(AnalysisTarget target) {
     return <String, TaskInput>{};
   }
 
   /**
    * Create a [GetContentTask] based on the given [target] in the given
    * [context].
    */
   static GetContentTask createTask(AnalysisContext context,
       AnalysisTarget target) {
     return new GetContentTask(context, target);
   }
 }
+
+/**
+ * A base class for analysis tasks whose target is expected to be a source.
+ */
+abstract class SourceBasedAnalysisTask extends AnalysisTask {
+  /**
+   * Initialize a newly created task to perform analysis within the given
+   * [context] in order to produce results for the given [target].
+   */
+  SourceBasedAnalysisTask(AnalysisContext context, AnalysisTarget target)
+      : super(context, target);
+
+  @override
+  String get description {
+    Source source = target.source;
+    String sourceName =
+        target.source == null ? '<unknown source>' : source.fullName;
+    return '${descriptor.name} for source $sourceName';
+  }
+}