Add assists to move Flutter widgets up and down one level

pkg/analysis_server/test/services/correction/assist_test.dart

Index: pkg/analysis_server/test/services/correction/assist_test.dart
diff --git a/pkg/analysis_server/test/services/correction/assist_test.dart b/pkg/analysis_server/test/services/correction/assist_test.dart
index 80a79399923bd100a464fc8866a8a6aeecb15649..b52c25d46d1f7a7d3eb1f39d103d442afbfffcff 100644
--- a/pkg/analysis_server/test/services/correction/assist_test.dart
+++ b/pkg/analysis_server/test/services/correction/assist_test.dart
@@ -54,9 +54,12 @@ class SingleChildRenderObjectWidget extends RenderObjectWidget {}
 class Transform extends SingleChildRenderObjectWidget {}
 class ClipRect extends SingleChildRenderObjectWidget { ClipRect.rect(){} }
 class AspectRatio extends SingleChildRenderObjectWidget {}
-class Container extends StatelessWidget { Container({child: null}){}}
+class Container extends StatelessWidget { Container({child: null, width: null, height: null}){}}
+class Center extends StatelessWidget { Center({child: null, key: null}){}}
 class DefaultTextStyle extends StatelessWidget { DefaultTextStyle({child: null}){}}
 class Row extends Widget { Row({children: null}){}}
+class GestureDetector extends SingleChildRenderObjectWidget { GestureDetector({child: null, onTap: null}){}}
+class Scaffold extends Widget { Scaffold({body: null}){}}
 ''';
 
   /**
@@ -3663,6 +3666,104 @@ class FakeFlutter {
     await assertNoAssist(DartAssistKind.REPARENT_FLUTTER_LIST);
   }
 
+  test_moveFlutterWidgetDown_OK() async {
+    _configureFlutterPkg({
+      'src/widgets/framework.dart': _flutter_framework_code,
+    });
+    await resolveTestUnit('''
+import 'package:flutter/src/widgets/framework.dart';
+build() {
+  return new Scaffold(
+// start
+    body: new /*caret*/GestureDetector(
+      onTap: () => startResize(),
+      child: new Center(
+        child: new Container(
+          width: 200.0,
+          height: 300.0,
+        ),
+        key: null,
+      ),
+    ),
+// end
+  );
+}
+startResize() {}
+''');
+    _setCaretLocation();
+    await assertHasAssist(
+        DartAssistKind.MOVE_FLUTTER_WIDGET_DOWN,
+        '''
+import 'package:flutter/src/widgets/framework.dart';
+build() {
+  return new Scaffold(
+// start
+    body: new Center(
+      child: new /*caret*/GestureDetector(
+        onTap: () => startResize(),
+        child: new Container(
+          width: 200.0,
+          height: 300.0,
+        ),
+      ),
+      key: null,
+    ),
+// end
+  );
+}
+startResize() {}
+''');
+  }
+
+  test_moveFlutterWidgetUp_OK() async {
+    _configureFlutterPkg({
+      'src/widgets/framework.dart': _flutter_framework_code,
+    });
+    await resolveTestUnit('''
+import 'package:flutter/src/widgets/framework.dart';
+build() {
+  return new Scaffold(
+// start
+    body: new Center(
+      child: new /*caret*/GestureDetector(
+        onTap: () => startResize(),
+        child: new Container(
+          width: 200.0,
+          height: 300.0,
+        ),
+      ),
+      key: null,
+    ),
+// end
+  );
+}
+startResize() {}
+''');
+    _setCaretLocation();
+    await assertHasAssist(
+        DartAssistKind.MOVE_FLUTTER_WIDGET_UP,
+        '''
+import 'package:flutter/src/widgets/framework.dart';
+build() {
+  return new Scaffold(
+// start
+    body: new /*caret*/GestureDetector(
+      onTap: () => startResize(),
+      child: new Center(
+        child: new Container(
+          width: 200.0,
+          height: 300.0,
+        ),
+        key: null,
+      ),
+    ),
+// end
+  );
+}
+startResize() {}
+''');
+  }
+
   test_reparentFlutterList_OK_multiLine() async {
     _configureFlutterPkg({
       'src/widgets/framework.dart': _flutter_framework_code,