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

pkg/analysis_server/test/protocol_test.dart

Index: pkg/analysis_server/test/protocol_test.dart
diff --git a/pkg/analysis_server/test/protocol_test.dart b/pkg/analysis_server/test/protocol_test.dart
index 06e795af73e2c08cb252a492987644e1d356f854..f8d55781296928a0baa2e187a1c24090dfe9808f 100644
--- a/pkg/analysis_server/test/protocol_test.dart
+++ b/pkg/analysis_server/test/protocol_test.dart
@@ -300,6 +300,16 @@ class RequestDatumTest {
     expect(indexResult.path, equals('myPath.bar'));
   }
 
+  @runTest
+  static void hasKey() {
+    var datum = makeDatum({
+      'foo': 'bar'
+    });
+    expect(datum.hasKey('foo'), isTrue);
+    expect(datum.hasKey('bar'), isFalse);
+    expect(datum.hasKey('baz'), isFalse);
+  }
+
   static void forEachMap_nonMap() {
     expect(() => makeDatum(1).forEachMap((key, value) {
       fail('Non-map should not be iterated');