Make these constants private

These constants aren't useful outside these files and are cause noise in the
dartdoc.

packages/flutter/lib/widgets/card.dart

@@ -6,7 +6,7 @@ import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/framework.dart';
 import 'package:sky/widgets/material.dart';
 
-const EdgeDims kCardMargins = const EdgeDims.all(4.0);
+const EdgeDims _kCardMargins = const EdgeDims.all(4.0);
 
 /// A material design card
 ///
@@ -19,7 +19,7 @@ class Card extends Component {
 
   Widget build() {
     return new Container(
-      margin: kCardMargins,
+      margin: _kCardMargins,
       child: new Material(
         color: color,
         type: MaterialType.card,

packages/flutter/lib/widgets/popup_menu_item.dart

@@ -10,8 +10,8 @@ import 'package:sky/widgets/gesture_detector.dart';
 import 'package:sky/widgets/ink_well.dart';
 import 'package:sky/widgets/theme.dart';
 
-const double kMenuItemHeight = 48.0;
-const double kBaselineOffsetFromBottom = 20.0;
+const double _kMenuItemHeight = 48.0;
+const double _kBaselineOffsetFromBottom = 20.0;
 
 class PopupMenuItem extends Component {
   PopupMenuItem({
@@ -30,11 +30,11 @@ class PopupMenuItem extends Component {
       onTap: onPressed,
       child: new InkWell(
         child: new Container(
-          height: kMenuItemHeight,
+          height: _kMenuItemHeight,
           child: new DefaultTextStyle(
             style: textStyle,
             child: new Baseline(
-              baseline: kMenuItemHeight - kBaselineOffsetFromBottom,
+              baseline: _kMenuItemHeight - _kBaselineOffsetFromBottom,
               child: child
             )
           )