[Cupertino] Implement nav_bar.

Runtime/cupertino/page_scaffold.cs

                     ? existingMediaQuery.viewInsets.copyWith(bottom: 0.0f)
                     : existingMediaQuery.viewInsets;
 
-                bool fullObstruction =
+                bool? fullObstruction =
-                if (fullObstruction) {
+                if (fullObstruction == true) {
                     paddedContent = new MediaQuery(
                         data: existingMediaQuery
                             .removePadding(removeTop: true)
     }
 
     public abstract class ObstructingPreferredSizeWidget : PreferredSizeWidget {
-        public bool fullObstruction { get; }
+
+        protected ObstructingPreferredSizeWidget(Key key = null) : base(key: key) {}
+        public virtual bool? fullObstruction { get; }
     }
 }

Runtime/cupertino/text_theme.cs

 
         readonly TextStyle _navTitleTextStyle;
 
-        TextStyle navTitleTextStyle {
+        public TextStyle navTitleTextStyle {
             get {
                 return this._navTitleTextStyle ??
                        (this._isLight

Runtime/widgets/transitions.cs

 using Unity.UIWidgets.ui;
 using UnityEngine;
 using Rect = Unity.UIWidgets.ui.Rect;
+using TextStyle = Unity.UIWidgets.painting.TextStyle;
 
 namespace Unity.UIWidgets.widgets {
     public abstract class AnimatedWidget : StatefulWidget {
                 alignment: this.alignment.value,
                 widthFactor: this.widthFactor,
                 heightFactor: this.heightFactor,
+                child: this.child
+            );
+        }
+    }
+
+    public class DefaultTextStyleTransition : AnimatedWidget {
+        public DefaultTextStyleTransition(
+            Key key = null,
+            Animation<TextStyle> style = null,
+            Widget child = null,
+            TextAlign? textAlign = null,
+            bool softWrap = true,
+            TextOverflow overflow = TextOverflow.clip,
+            int? maxLines = null
+        ) : base(key: key, listenable: style) {
+            D.assert(style != null);
+            D.assert(child != null);
+            this.textAlign = textAlign;
+            this.softWrap = softWrap;
+            this.overflow = overflow;
+            this.maxLines = maxLines;
+            this.child = child;
+        }
+
+        Animation<TextStyle> style {
+            get { return (Animation<TextStyle>) this.listenable; }
+        }
+
+        public readonly TextAlign? textAlign;
+
+        public readonly bool softWrap;
+        public readonly TextOverflow overflow;
+        public readonly int? maxLines;
+        public readonly Widget child;
+
+        protected internal override Widget build(BuildContext context) {
+            return new DefaultTextStyle(
+                style: this.style.value,
+                textAlign: this.textAlign,
+                softWrap: this.softWrap,
+                overflow: this.overflow,
+                maxLines: this.maxLines,
                 child: this.child
             );
         }

Samples/UIWidgetsGallery/demo/animation/home.cs

             );
         }
 
-        protected override Widget build(BuildContext context) {
+        protected internal override Widget build(BuildContext context) {
             return new LayoutBuilder(builder: this._build);
         }
     }

Samples/UIWidgetsGallery/demo/material/backdrop_demo.cs

         ) : base(key: key, listenable: listenable) {
         }
 
-        protected override Widget build(BuildContext context) {
+        protected internal override Widget build(BuildContext context) {
             Animation<float> animation = (Animation<float>) this.listenable;
             return new DefaultTextStyle(
                 style: Theme.of(context).primaryTextTheme.title,

Samples/UIWidgetsGallery/gallery/backdrop.cs

         public readonly Widget child0;
         public readonly Widget child1;
 
-        protected override Widget build(BuildContext context) {
+        protected internal override Widget build(BuildContext context) {
             Animation<float> progress = this.listenable as Animation<float>;
 
             float opacity1 = new CurvedAnimation(

Runtime/widgets/value_listenable_builder.cs

+using Unity.UIWidgets.foundation;
+
+namespace Unity.UIWidgets.widgets {
+    public delegate Widget ValueWidgetBuilder<T>(BuildContext context, T value, Widget child);
+
+    public class ValueListenableBuilder<T> : StatefulWidget {
+        public ValueListenableBuilder(
+            ValueListenable<T> valueListenable,
+            ValueWidgetBuilder<T> builder,
+            Widget child = null
+        ) {
+            D.assert(valueListenable != null);
+            D.assert(builder != null);
+            this.valueListenable = valueListenable;
+            this.builder = builder;
+            this.child = child;
+        }
+
+        public readonly ValueListenable<T> valueListenable;
+
+        public readonly ValueWidgetBuilder<T> builder;
+
+        public readonly Widget child;
+
+        public override State createState() {
+            return new _ValueListenableBuilderState<T>();
+        }
+    }
+
+    class _ValueListenableBuilderState<T> : State<ValueListenableBuilder<T>> {
+        T value;
+
+        public override void initState() {
+            base.initState();
+            this.value = this.widget.valueListenable.value;
+            this.widget.valueListenable.addListener(this._valueChanged);
+        }
+
+        public override void didUpdateWidget(StatefulWidget _oldWidget) {
+            ValueListenableBuilder<T> oldWidget = _oldWidget as ValueListenableBuilder<T>;
+            if (oldWidget.valueListenable != this.widget.valueListenable) {
+                oldWidget.valueListenable.removeListener(this._valueChanged);
+                this.value = this.widget.valueListenable.value;
+                this.widget.valueListenable.addListener(this._valueChanged);
+            }
+
+            base.didUpdateWidget(oldWidget);
+        }
+
+        public override void dispose() {
+            this.widget.valueListenable.removeListener(this._valueChanged);
+            base.dispose();
+        }
+
+        void _valueChanged() {
+            this.setState(() => { this.value = this.widget.valueListenable.value; });
+        }
+
+        public override Widget build(BuildContext context) {
+            return this.widget.builder(context, this.value, this.widget.child);
+        }
+    }
+}

Runtime/widgets/value_listenable_builder.cs.meta

+fileFormatVersion: 2
+guid: c4b7ae22b588cdf4093501087a4ecadb
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: