allow client to specify any combination of add/modified/removed

pkg/analysis_server/lib/src/protocol.dart

Index: pkg/analysis_server/lib/src/protocol.dart
diff --git a/pkg/analysis_server/lib/src/protocol.dart b/pkg/analysis_server/lib/src/protocol.dart
index 7cdfa18de0c22eb00ea302288a470cda0cc29a96..2c4ecfb258e5aca1dddae53b172134cf60e7ab05 100644
--- a/pkg/analysis_server/lib/src/protocol.dart
+++ b/pkg/analysis_server/lib/src/protocol.dart
@@ -185,6 +185,17 @@ class RequestDatum {
   }
 
   /**
+   * Return `true` if the datum is a Map containing the given [key].
+   */
+  bool hasKey(String key) {
+    if (datum is! Map<String, dynamic>) {
+      throw new RequestFailure(new Response.invalidParameter(request, path,
+          "be a map"));
+    }
+    return datum.containsKey(key);
+  }
+
+  /**
    * Validate that the datum is a Map whose keys are strings, and call [f] on
    * each key/value pair in the map.
    */