Compare SemanticsHandler# at end of test to value at beginning of test (#18183)

packages/flutter/lib/src/rendering/object.dart

diff --git a/packages/flutter/lib/src/rendering/object.dart b/packages/flutter/lib/src/rendering/object.dart
index abca235..66b5759 100644
--- a/packages/flutter/lib/src/rendering/object.dart
+++ b/packages/flutter/lib/src/rendering/object.dart
@@ -822,7 +822,12 @@
   SemanticsOwner get semanticsOwner => _semanticsOwner;
   SemanticsOwner _semanticsOwner;
 
-  int _outstandingSemanticsHandle = 0;
+  /// The number of clients registered to listen for semantics.
+  ///
+  /// The number is increased whenever [ensureSemantics] is called and decreased
+  /// when [SemanticsHandle.dispose] is called.
+  int get debugOutstandingSemanticsHandles => _outstandingSemanticsHandles;
+  int _outstandingSemanticsHandles = 0;
 
   /// Opens a [SemanticsHandle] and calls [listener] whenever the semantics tree
   /// updates.
@@ -837,8 +842,8 @@
   /// objects for a given [PipelineOwner] are closed, the [PipelineOwner] stops
   /// maintaining the semantics tree.
   SemanticsHandle ensureSemantics({ VoidCallback listener }) {
-    _outstandingSemanticsHandle += 1;
-    if (_outstandingSemanticsHandle == 1) {
+    _outstandingSemanticsHandles += 1;
+    if (_outstandingSemanticsHandles == 1) {
       assert(_semanticsOwner == null);
       _semanticsOwner = new SemanticsOwner();
       if (onSemanticsOwnerCreated != null)
@@ -849,8 +854,8 @@
 
   void _didDisposeSemanticsHandle() {
     assert(_semanticsOwner != null);
-    _outstandingSemanticsHandle -= 1;
-    if (_outstandingSemanticsHandle == 0) {
+    _outstandingSemanticsHandles -= 1;
+    if (_outstandingSemanticsHandles == 0) {
       _semanticsOwner.dispose();
       _semanticsOwner = null;
       if (onSemanticsOwnerDisposed != null)

packages/flutter_test/lib/src/widget_tester.dart

diff --git a/packages/flutter_test/lib/src/widget_tester.dart b/packages/flutter_test/lib/src/widget_tester.dart
index f8d8ee7..53bf6ea 100644
--- a/packages/flutter_test/lib/src/widget_tester.dart
+++ b/packages/flutter_test/lib/src/widget_tester.dart
@@ -57,9 +57,10 @@
   final TestWidgetsFlutterBinding binding = TestWidgetsFlutterBinding.ensureInitialized();
   final WidgetTester tester = new WidgetTester._(binding);
   timeout ??= binding.defaultTestTimeout;
-    test_package.test(
+  test_package.test(
     description,
     () {
+      tester._recordNumberOfSemanticsHandles();
       test_package.addTearDown(binding.postTest);
       return binding.runTest(
         () => callback(tester),
@@ -125,6 +126,7 @@
   final TestWidgetsFlutterBinding binding = TestWidgetsFlutterBinding.ensureInitialized();
   assert(binding is! AutomatedTestWidgetsFlutterBinding);
   final WidgetTester tester = new WidgetTester._(binding);
+  tester._recordNumberOfSemanticsHandles();
   return binding.runTest(
     () => callback(tester),
     tester._endOfTestVerifications,
@@ -523,7 +525,8 @@
   }
 
   void _verifySemanticsHandlesWereDisposed() {
-    if (binding.pipelineOwner.semanticsOwner != null) {
+    assert(_lastRecordedSemanticsHandles != null);
+    if (binding.pipelineOwner.debugOutstandingSemanticsHandles > _lastRecordedSemanticsHandles) {
       throw new FlutterError(
         'A SemanticsHandle was active at the end of the test.\n'
         'All SemanticsHandle instances must be disposed by calling dispose() on '
@@ -532,6 +535,13 @@
         'existing handle will leak into another test and alter its behavior.'
       );
     }
+    _lastRecordedSemanticsHandles = null;
+  }
+
+  int _lastRecordedSemanticsHandles;
+
+  void _recordNumberOfSemanticsHandles() {
+    _lastRecordedSemanticsHandles = binding.pipelineOwner.debugOutstandingSemanticsHandles;
   }
 
   /// Returns the TestTextInput singleton.