Keep text selection ranges within the length of the text (#11055)

Fixes https://github.com/flutter/flutter/issues/11041

packages/flutter/lib/src/widgets/editable_text.dart

@@ -69,7 +69,9 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   /// this value should only be set between frames, e.g. in response to user
   /// actions, not during the build, layout, or paint phases.
   set text(String newText) {
-    value = value.copyWith(text: newText, composing: TextRange.empty);
+    value = value.copyWith(text: newText,
+                           selection: const TextSelection.collapsed(offset: -1),
+                           composing: TextRange.empty);
   }
 
   /// The currently selected [text].
@@ -82,6 +84,8 @@ class TextEditingController extends ValueNotifier<TextEditingValue> {
   /// this value should only be set between frames, e.g. in response to user
   /// actions, not during the build, layout, or paint phases.
   set selection(TextSelection newSelection) {
+    if (newSelection.start > text.length || newSelection.end > text.length)
+      throw new FlutterError('invalid text selection: $newSelection');
     value = value.copyWith(selection: newSelection, composing: TextRange.empty);
   }
 

packages/flutter/test/material/text_field_test.dart

@@ -1574,4 +1574,22 @@ void main() {
       expect(controller1.selection, equals(TextRange.empty));
     }
   );
+
+  testWidgets(
+    'Selection is consistent with text length',
+    (WidgetTester tester) async {
+      final TextEditingController controller = new TextEditingController();
+
+      controller.text = 'abcde';
+      controller.selection = const TextSelection.collapsed(offset: 5);
+
+      controller.text = '';
+      expect(controller.selection.start, lessThanOrEqualTo(0));
+      expect(controller.selection.end, lessThanOrEqualTo(0));
+
+      expect(() {
+        controller.selection = const TextSelection.collapsed(offset: 10);
+      }, throwsFlutterError);
+    }
+  );
 }