Nnbd widgets (#64672)

* migrate widget to nullsafety

* remove double blank line after license

* address review comments in actions.dart

* nullable ObjectKey.value

* use local variable oldElement

* make State.build non-nullable

* make State.context non-nullable

* newline at eof

* make ProxyWidget.child non-nullable

* make _InactiveElements.debugContains non-nullable

* make Element.depth non-nullable

* make ProxyElement.build non-nullable

* make StatefulElement.state non-nullable

* remove 'Notice that'

* avoid cast of list in RenderObjectElement.updateChildren

* make IndexedSlot.value non-nullable

* avoid cast of list in MultiChildRenderObjectElement.mount

* make some WidgetsApp parameters non-nullable

* hitTest take non-nullable position

* make ScrollableState.position non-nullable

* use _pixels instead of pixels

* make ViewportOffset.pixels non-nullable

* make param and return type of IndexedWidgetBuilder non-nullable

* unused_import

* make context param non-nullable for Builder in animated_list.dart

* make ScrollMetrics.viewportDimension non-nullable

* make ScrollMetrics.{min,max}ScrollExtent non-nullable

* make _Location.file non-nullable

* _WidgetForTypeTests.createElement throw UnimplementedError

* update _NullWidget.build error message

* make _ShortcutsState.manager non-nullable

* Fix childCount issues for NNBD

* fix childCount computation on web

* increase max value on js side to compute childCount

* make aspect parameter of dependOnInheritedWidgetOfExactType nullable

* merge has{min,max}ScrollExtent into hasScrollExtents

* update focus_manager.dart

* address review comments in icon.dart

* address review comments in image.dart

* address review comments in routes.dart

* address review comments in scroll_activity.dart

* update doc comments

* make UserScrollNotification.direction non-nullable and required

* rename hasScrollExtents to hasContentDimensions

* unnecessary late

Co-authored-by: Ian Hickson <ian@hixie.ch>

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

@@ -645,7 +645,7 @@ class CupertinoTextField extends StatefulWidget {
   }
 }
 
-class _CupertinoTextFieldState extends State<CupertinoTextField> with RestorationMixin, AutomaticKeepAliveClientMixin implements TextSelectionGestureDetectorBuilderDelegate {
+class _CupertinoTextFieldState extends State<CupertinoTextField> with RestorationMixin, AutomaticKeepAliveClientMixin<CupertinoTextField> implements TextSelectionGestureDetectorBuilderDelegate {
   final GlobalKey _clearGlobalKey = GlobalKey();
 
   RestorableTextEditingController _controller;

packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart

@@ -196,28 +196,11 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
     if (firstChild == null) {
       if (!addInitialChild(index: firstIndex, layoutOffset: indexToLayoutOffset(itemExtent, firstIndex))) {
         // There are either no children, or we are past the end of all our children.
-        // If it is the latter, we will need to find the first available child.
         double max;
-        if (childManager.childCount != null) {
-          max = computeMaxScrollOffset(constraints, itemExtent);
-          // TODO(ianh): null-aware flow analysis flags the next two
-          // branches as entirely dead code, and it's hard to argue with
-          // its logic.
-        } else if (firstIndex <= 0) { // ignore: dead_code
+        if (firstIndex <= 0) {
           max = 0.0;
         } else {
-          // We will have to find it manually.
-          int possibleFirstIndex = firstIndex - 1;
-          while (
-            possibleFirstIndex > 0 &&
-            !addInitialChild(
-              index: possibleFirstIndex,
-              layoutOffset: indexToLayoutOffset(itemExtent, possibleFirstIndex),
-            )
-          ) {
-            possibleFirstIndex -= 1;
-          }
-          max = (possibleFirstIndex + 1) * itemExtent;
+          max = computeMaxScrollOffset(constraints, itemExtent);
         }
         geometry = SliverGeometry(
           scrollExtent: max,

packages/flutter/lib/src/rendering/viewport_offset.dart

@@ -94,6 +94,9 @@ abstract class ViewportOffset extends ChangeNotifier {
   /// the value changes due to [correctBy]).
   double get pixels;
 
+  /// Whether the [pixels] property is available.
+  bool get hasPixels;
+
   /// Called when the viewport's extents are established.
   ///
   /// The argument is the dimension of the [RenderViewport] in the main axis
@@ -245,7 +248,9 @@ abstract class ViewportOffset extends ChangeNotifier {
   /// `super.debugFillDescription(description)`.
   @mustCallSuper
   void debugFillDescription(List<String> description) {
-    description.add('offset: ${pixels.toStringAsFixed(1)}');
+    if (hasPixels) {
+      description.add('offset: ${pixels.toStringAsFixed(1)}');
+    }
   }
 }
 
@@ -258,6 +263,9 @@ class _FixedViewportOffset extends ViewportOffset {
   @override
   double get pixels => _pixels;
 
+  @override
+  bool get hasPixels => true;
+
   @override
   bool applyViewportDimension(double viewportDimension) => true;