Popup menu splash doesn't go to edge

Now the splash goes to the edge. Also, fold popup_menu_item.dart into
popup_menu.dart for simplicity.

Fixes flutter#266

Author: abarth

packages/flutter/lib/material.dart

@@ -37,7 +37,6 @@ export 'src/material/material_button.dart';
 export 'src/material/material_list.dart';
 export 'src/material/page.dart';
 export 'src/material/popup_menu.dart';
-export 'src/material/popup_menu_item.dart';
 export 'src/material/progress_indicator.dart';
 export 'src/material/radio.dart';
 export 'src/material/raised_button.dart';

packages/flutter/lib/src/material/popup_menu.dart

@@ -9,15 +9,42 @@ import 'package:flutter/widgets.dart';
 
 import 'ink_well.dart';
 import 'material.dart';
-import 'popup_menu_item.dart';
+import 'theme.dart';
 
 const Duration _kMenuDuration = const Duration(milliseconds: 300);
+const double _kBaselineOffsetFromBottom = 20.0;
 const double _kMenuCloseIntervalEnd = 2.0 / 3.0;
-const double _kMenuWidthStep = 56.0;
-const double _kMenuMinWidth = 2.0 * _kMenuWidthStep;
-const double _kMenuMaxWidth = 5.0 * _kMenuWidthStep;
 const double _kMenuHorizontalPadding = 16.0;
+const double _kMenuItemHeight = 48.0;
+const double _kMenuMaxWidth = 5.0 * _kMenuWidthStep;
+const double _kMenuMinWidth = 2.0 * _kMenuWidthStep;
 const double _kMenuVerticalPadding = 8.0;
+const double _kMenuWidthStep = 56.0;
+
+class PopupMenuItem<T> extends StatelessComponent {
+  PopupMenuItem({
+    Key key,
+    this.value,
+    this.child
+  }) : super(key: key);
+
+  final Widget child;
+  final T value;
+
+  Widget build(BuildContext context) {
+    return new Container(
+      height: _kMenuItemHeight,
+      padding: const EdgeDims.symmetric(horizontal: _kMenuHorizontalPadding),
+      child: new DefaultTextStyle(
+        style: Theme.of(context).text.subhead,
+        child: new Baseline(
+          baseline: _kMenuItemHeight - _kBaselineOffsetFromBottom,
+          child: child
+        )
+      )
+    );
+  }
+}
 
 class _PopupMenu<T> extends StatelessComponent {
   _PopupMenu({
@@ -71,7 +98,6 @@ class _PopupMenu<T> extends StatelessComponent {
                   child: new Block(
                     children,
                     padding: const EdgeDims.symmetric(
-                      horizontal: _kMenuHorizontalPadding,
                       vertical: _kMenuVerticalPadding
                     )
                   )