Add TODOs to remove lint ignore directives. (#21170)

This is follow-up to https://github.com/flutter/flutter/pull/21143.

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

@@ -51,6 +51,7 @@ class AndroidView extends StatefulWidget {
   /// The `viewType`, `hitTestBehavior`, and `gestureRecognizers` parameters must not be null.
   /// If `creationParams` is not null then `creationParamsCodec` must not be null.
   AndroidView({ // ignore: prefer_const_constructors_in_immutables
+                // TODO(aam): Remove lint ignore above once dartbug.com/34297 is fixed
     Key key,
     @required this.viewType,
     this.onPlatformViewCreated,

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

@@ -51,6 +51,7 @@ abstract class ScrollView extends StatelessWidget {
   ///
   /// If the [primary] argument is true, the [controller] must be null.
   ScrollView({ // ignore: prefer_const_constructors_in_immutables
+               // TODO(aam): Remove lint ignore above once dartbug.com/34297 is fixed
     Key key,
     this.scrollDirection = Axis.vertical,
     this.reverse = false,

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

@@ -184,6 +184,7 @@ import 'scrollable.dart';
 class SingleChildScrollView extends StatelessWidget {
   /// Creates a box in which a single widget can be scrolled.
   SingleChildScrollView({ // ignore: prefer_const_constructors_in_immutables
+                          // TODO(aam): Remove lint ignore above once dartbug.com/34297 is fixed
     Key key,
     this.scrollDirection = Axis.vertical,
     this.reverse = false,