Add simple Dart tasks

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

Index: pkg/analyzer/lib/src/task/general.dart
diff --git a/pkg/analyzer/lib/src/task/general.dart b/pkg/analyzer/lib/src/task/general.dart
index 25feb6f6d320ce397fd92636cbb085fd2d089647..5cb31ed699b0a0a3bfb9af9389ad44d9ad5d51bf 100644
--- a/pkg/analyzer/lib/src/task/general.dart
+++ b/pkg/analyzer/lib/src/task/general.dart
@@ -13,7 +13,7 @@ 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.
    */
@@ -31,15 +31,6 @@ class GetContentTask extends AnalysisTask {
       : 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
@@ -68,3 +59,23 @@ class GetContentTask extends AnalysisTask {
     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';
+  }
+}