Generate useful toString() methods for summary classes.

pkg/analyzer/lib/src/summary/base.dart

Index: pkg/analyzer/lib/src/summary/base.dart
diff --git a/pkg/analyzer/lib/src/summary/base.dart b/pkg/analyzer/lib/src/summary/base.dart
index c4a4cdb8a6aeac9b622bd2099f44e0842a69574a..1a1626ee9d65cee483e19f7c77efa4678d7e65c6 100644
--- a/pkg/analyzer/lib/src/summary/base.dart
+++ b/pkg/analyzer/lib/src/summary/base.dart
@@ -33,4 +33,15 @@ abstract class SummaryClass {
    * Intended for testing and debugging only.
    */
   Map<String, Object> toMap();
+
+  /**
+   * Translate the data in this class into a JSON map, whose keys are the names
+   * of fields and whose values are the data stored in those fields,
+   * recursively converted into JSON.
+   *
+   * Fields containing their default value are elided.
+   *
+   * Intended for testing and debugging only.
+   */
+  Map<String, Object> toJson();
 }