Use JavaScript to detect if Selection.modify is available.

dart/site/try/src/settings.dart

Index: dart/site/try/src/settings.dart
diff --git a/dart/site/try/src/settings.dart b/dart/site/try/src/settings.dart
index b78aa53924f42a1b1e09c0022fa29ef3a4015a9b..52a7cc2c976d8fb72622d76c30f1197be106f11c 100644
--- a/dart/site/try/src/settings.dart
+++ b/dart/site/try/src/settings.dart
@@ -101,6 +101,9 @@ const BooleanUserOption alwaysRunInIframe =
 const BooleanUserOption communicateViaBlobs =
     const BooleanUserOption('communicateViaBlobs', isHidden: true);
 
+const BooleanUserOption hasSelectionModify =
+    const BooleanUserOption('hasSelectionModify', isHidden: true);
+
 const List<UserOption> options = const <UserOption>[
     _alwaysRunInWorker,
     _verboseCompiler,
@@ -116,4 +119,5 @@ const List<UserOption> options = const <UserOption>[
     _currentSample,
     alwaysRunInIframe,
     communicateViaBlobs,
+    hasSelectionModify,
   ];