loading button should not be transparent

components/button/demo/loading.md

@@ -24,15 +24,15 @@ const App = React.createClass({
   render() {
     return (
       <div>
-        <Button type="primary" size="large" loading>
-          加载中
-        </Button>
         <Button type="primary" loading>
           加载中
         </Button>
         <Button type="primary" size="small" loading>
           加载中
         </Button>
+        <Button loading>
+          加载中
+        </Button>
         <br />
         <Button type="primary" loading={this.state.loading} onClick={this.enterLoading}>
           点击变加载

style/components/button.less

@@ -46,6 +46,22 @@
     .btn-circle(@btn-prefix-cls);
   }
 
+  &:before {
+    position: absolute;
+    top: -1px;
+    left: -1px;
+    bottom: -1px;
+    right: -1px;
+    background: #fff;
+    opacity: 0.35;
+    content: '';
+    border-radius: inherit;
+    z-index: 1;
+    transition: opacity .2s;
+    pointer-events: none;
+    display: none;
+  }
+
   &:after {
     font-family: anticon;
     content: "\e6a1";
@@ -58,11 +74,14 @@
   &&-loading {
     padding-left: 29px;
     pointer-events: none;
-    opacity: 0.75;
+    position: relative;
     .anticon {
       margin-left: -14px;
       .transition(all .3s @ease-in-out);
     }
+    &:before {
+      display: block;
+    }
   }
 
   &-sm&-loading {