Update expansion tile and flexible space bar. Fix bottom app bar demo.

Runtime/material/expansion_tile.cs

 
         Widget _buildChildren(BuildContext context, Widget child) {
             Color borderSideColor = this._borderColor.value ?? Colors.transparent;
-            Color titleColor = this._headerColor.value;
 
             return new Container(
                 decoration: new BoxDecoration(
                 child: new Column(
                     mainAxisSize: MainAxisSize.min,
                     children: new List<Widget> {
-                        IconTheme.merge(
-                            data: new IconThemeData(color: this._iconColor.value),
+                        ListTileTheme.merge(
+                            iconColor: this._iconColor.value,
+                            textColor: this._headerColor.value,
-                                title: new DefaultTextStyle(
-                                    style: Theme.of(this.context).textTheme.subhead.copyWith(color: titleColor),
-                                    child: this.widget.title
-                                ),
+                                title: this.widget.title,
                                 trailing: this.widget.trailing ?? new RotationTransition(
                                               turns: this._iconTurns,
                                               child: new Icon(Icons.expand_more)

Runtime/material/flexible_space_bar.cs

             Widget title = null,
             Widget background = null,
             bool? centerTitle = null,
+            EdgeInsets titlePadding = null,
+            this.titlePadding = titlePadding;
             this.collapseMode = collapseMode;
         }
 
 
         public readonly bool? centerTitle;
-
+        
+
+        public readonly EdgeInsets titlePadding;
 
         public static Widget createSettings(
             float? toolbarOpacity = null,
                     color: titleStyle.color.withOpacity(toolbarOpacity));
 
                 bool effectiveCenterTitle = this._getEffectiveCenterTitle(theme).Value;
+                EdgeInsets padding = this.widget.titlePadding ??
+                    EdgeInsets.only(
+                        left: effectiveCenterTitle ? 0.0f : 72.0f,
+                        bottom: 16.0f
+                    );
-                        padding: EdgeInsets.fromLTRB(
-                            effectiveCenterTitle ? 0.0f : 72.0f,
-                            0f,
-                            0f,
-                            16.0f),
+                        padding: padding,
                         child: new Transform(
                             alignment: titleAlignment,
                             transform: scaleTransform,

Samples/UIWidgetsGallery/demo/material/bottom_app_bar_demo.cs

         }
 
         public override Path getOuterPath(Rect host, Rect guest) {
-            if (!host.overlaps(guest)) {
+            if (guest == null || !host.overlaps(guest)) {
                 Path path = new Path();
                 path.addRect(host);
                 return path;