Clear the selection when a text widget loses focus (flutter#10938)

Fixes flutter#10911

Author: jason-simmons

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

@@ -531,6 +531,10 @@ class EditableTextState extends State<EditableText> implements TextInputClient {
     _openOrCloseInputConnectionIfNeeded();
     _startOrStopCursorTimerIfNeeded();
     _updateOrDisposeSelectionOverlayIfNeeded();
+    if (!_hasFocus) {
+      // Clear the selection and composition state if this widget lost focus.
+      _value = new TextEditingValue(text: _value.text);
+    }
   }
 
   @override

packages/flutter/test/material/text_field_test.dart

@@ -1503,4 +1503,41 @@ void main() {
       expect(scrollableState.position.pixels, isNot(equals(0.0)));
     }
   );
+
+  testWidgets(
+    'Text field drops selection when losing focus',
+    (WidgetTester tester) async {
+      final Key key1 = new UniqueKey();
+      final TextEditingController controller1 = new TextEditingController();
+      final Key key2 = new UniqueKey();
+
+      Widget builder() {
+        return overlay(new Center(
+          child: new Material(
+            child: new Column(
+              children: <Widget>[
+                new TextField(
+                  key: key1,
+                  controller: controller1
+                ),
+                new TextField(key: key2),
+              ],
+            ),
+          ),
+        ));
+      }
+
+      await tester.pumpWidget(builder());
+      await tester.tap(find.byKey(key1));
+      await tester.enterText(find.byKey(key1), 'abcd');
+      await tester.pump();
+      controller1.selection = const TextSelection(baseOffset: 0, extentOffset: 3);
+      await tester.pump();
+      expect(controller1.selection, isNot(equals(TextRange.empty)));
+
+      await tester.tap(find.byKey(key2));
+      await tester.pump();
+      expect(controller1.selection, equals(TextRange.empty));
+    }
+  );
 }