Correct handling for nulling TextField controller (#12190)

Correctly handle the case where the TextEditingController associated
with a TextField is set to null. Due to a typo (= vs ==), previously
this was a no-op and the existing TextEditingController was retained,
rather than copied.

packages/flutter/lib/src/material/text_field.dart

@@ -213,7 +213,7 @@ class _TextFieldState extends State<TextField> {
   void didUpdateWidget(TextField oldWidget) {
     super.didUpdateWidget(oldWidget);
     if (widget.controller == null && oldWidget.controller != null)
-      _controller == new TextEditingController.fromValue(oldWidget.controller.value);
+      _controller = new TextEditingController.fromValue(oldWidget.controller.value);
     else if (widget.controller != null && oldWidget.controller == null)
       _controller = null;
   }

packages/flutter/test/material/text_field_test.dart

@@ -1233,14 +1233,14 @@ void main() {
   });
 
   testWidgets('Controller can update server', (WidgetTester tester) async {
-    final TextEditingController controller = new TextEditingController(
+    final TextEditingController controller1 = new TextEditingController(
       text: 'Initial Text',
     );
     final TextEditingController controller2 = new TextEditingController(
       text: 'More Text',
     );
 
-    TextEditingController currentController = controller;
+    TextEditingController currentController;
     StateSetter setState;
 
     await tester.pumpWidget(
@@ -1255,36 +1255,64 @@ void main() {
     );
     expect(tester.testTextInput.editingState['text'], isEmpty);
 
+    // Initial state with null controller.
     await tester.tap(find.byType(TextField));
     await tester.pump();
+    expect(tester.testTextInput.editingState['text'], isEmpty);
+
+    // Update the controller from null to controller1.
+    setState(() {
+      currentController = controller1;
+    });
+    await tester.pump();
     expect(tester.testTextInput.editingState['text'], equals('Initial Text'));
 
-    controller.text = 'Updated Text';
+    // Verify that updates to controller1 are handled.
+    controller1.text = 'Updated Text';
     await tester.idle();
     expect(tester.testTextInput.editingState['text'], equals('Updated Text'));
 
+    // Verify that switching from controller1 to controller2 is handled.
     setState(() {
       currentController = controller2;
     });
     await tester.pump();
     expect(tester.testTextInput.editingState['text'], equals('More Text'));
 
-    controller.text = 'Ignored Text';
+    // Verify that updates to controller1 are ignored.
+    controller1.text = 'Ignored Text';
     await tester.idle();
     expect(tester.testTextInput.editingState['text'], equals('More Text'));
 
+    // Verify that updates to controller text are handled.
     controller2.text = 'Additional Text';
     await tester.idle();
     expect(tester.testTextInput.editingState['text'], equals('Additional Text'));
 
+    // Verify that updates to controller selection are handled.
     controller2.selection = const TextSelection(baseOffset: 0, extentOffset: 5);
     await tester.idle();
     expect(tester.testTextInput.editingState['selectionBase'], equals(0));
     expect(tester.testTextInput.editingState['selectionExtent'], equals(5));
 
+    // Verify that calling clear() clears the text.
     controller2.clear();
     await tester.idle();
     expect(tester.testTextInput.editingState['text'], equals(''));
+
+    // Verify that switching from controller2 to null preserves current text.
+    controller2.text = 'The Final Cut';
+    await tester.idle();
+    expect(tester.testTextInput.editingState['text'], equals('The Final Cut'));
+    setState(() {
+      currentController = null;
+    });
+    await tester.pump();
+    expect(tester.testTextInput.editingState['text'], equals('The Final Cut'));
+
+    // Verify that changes to controller2 are ignored.
+    controller2.text = 'Goodbye Cruel World';
+    expect(tester.testTextInput.editingState['text'], equals('The Final Cut'));
   });
 
   testWidgets('Cannot enter new lines onto single line TextField', (WidgetTester tester) async {