- Roll engine to version f9ebf21 (flutter#6839)

* - Roll engine to version f9ebf21
- Update @pragma directive for some classes in semantics.dart

* Update license file.

DEPS

@@ -31,7 +31,7 @@ vars = {
   # Dart is: https://github.com/dart-lang/sdk/blob/master/DEPS.
   # You can use //tools/dart/create_updated_flutter_deps.py to produce
   # updated revision list of existing dependencies.
-  'dart_revision': '9c07fb64c48adb3d6fde50bab6b8b641c5b67683',
+  'dart_revision': 'f9ebf2129732fd2b606286fdf58e500384b8a0bc',
 
   # WARNING: DO NOT EDIT MANUALLY
   # The lines between blank lines above and below are generated by a script. See create_updated_flutter_deps.py

ci/licenses_golden/licenses_third_party

@@ -1,4 +1,4 @@
-Signature: fc569d77237ef4247eb7a93bb2c989be
+Signature: 688d05f1b8d8726e65b28a27ccc7dfcb
 
 UNUSED LICENSES:
 

lib/ui/semantics.dart

@@ -550,6 +550,7 @@ class SemanticsFlag {
 ///
 /// Once created, the [SemanticsUpdate] objects can be passed to
 /// [Window.updateSemantics] to update the semantics conveyed to the user.
+@pragma('vm:entry-point')
 class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
   /// Creates an empty [SemanticsUpdateBuilder] object.
   @pragma('vm:entry-point')
@@ -723,6 +724,7 @@ class SemanticsUpdateBuilder extends NativeFieldWrapperClass2 {
 ///
 /// Semantics updates can be applied to the system's retained semantics tree
 /// using the [Window.updateSemantics] method.
+@pragma('vm:entry-point')
 class SemanticsUpdate extends NativeFieldWrapperClass2 {
   /// This class is created by the engine, and should not be instantiated
   /// or extended directly.