Convert some Columns into Blocks (#3210)

These columns were secretly re-creating Block in a more complex way. Now we
just use Block directly.

examples/material_gallery/lib/demo/full_screen_dialog_demo.dart

@@ -159,12 +159,8 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
           )
         ]
       ),
-      body: new Padding(
+      body: new Block(
         padding: const EdgeInsets.all(16.0),
-        child: new ScrollableViewport(
-          child: new Column(
-            crossAxisAlignment: CrossAxisAlignment.stretch,
-            mainAxisAlignment: MainAxisAlignment.collapse,
         children: <Widget>[
           new Container(
             padding: const EdgeInsets.symmetric(vertical: 8.0),
@@ -187,8 +183,7 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
             )
           ),
           new Column(
-                crossAxisAlignment: CrossAxisAlignment.stretch,
-                mainAxisAlignment: MainAxisAlignment.end,
+            crossAxisAlignment: CrossAxisAlignment.start,
             children: <Widget>[
               new Text('From', style: theme.textTheme.caption),
               new DateTimeItem(
@@ -203,8 +198,7 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
             ]
           ),
           new Column(
-                crossAxisAlignment: CrossAxisAlignment.stretch,
-                mainAxisAlignment: MainAxisAlignment.end,
+            crossAxisAlignment: CrossAxisAlignment.start,
             children: <Widget>[
               new Text('To', style: theme.textTheme.caption),
               new DateTimeItem(
@@ -247,8 +241,6 @@ class FullScreenDialogDemoState extends State<FullScreenDialogDemo> {
         })
         .toList()
       )
-        )
-      )
     );
   }
 }

examples/material_gallery/lib/demo/list_demo.dart

@@ -38,9 +38,7 @@ class ListDemoState extends State<ListDemo> {
         decoration: new BoxDecoration(
           border: new Border(top: new BorderSide(color: Colors.black26))
         ),
-        child: new Column(
-          mainAxisAlignment: MainAxisAlignment.collapse,
-          crossAxisAlignment: CrossAxisAlignment.stretch,
+        child: new Block(
           children: <Widget>[
             new ListItem(
               dense: true,

examples/material_gallery/lib/demo/tooltip_demo.dart

@@ -19,8 +19,7 @@ class TooltipDemo extends StatelessWidget {
       ),
       body: new Builder(
         builder: (BuildContext context) {
-          return new Column(
-            crossAxisAlignment: CrossAxisAlignment.stretch,
+          return new Block(
             children: <Widget>[
               new Text(_introText, style: theme.textTheme.subhead),
               new Row(