部分svg图形无法以symbol的方式预览？
部分svg的图像在导出后以symbol模式无法正常预览，可能是因为linearGradient标签？无渐变的图像没有碰到这样的问题，供测试的一个图形：
```html
<svg width="80" height="80" 
  xmlns="http://www.w3.org/2000/svg" 
  xmlns:xlink="http://www.w3.org/1999/xlink">
  <defs>
    <filter x="-56.1%" y="-47.1%" width="212.1%" height="236.4%" filterUnits="objectBoundingBox" id="a">
      <feOffset dy="11" in="SourceAlpha" result="shadowOffsetOuter1"/>
      <feGaussianBlur stdDeviation="10.5" in="shadowOffsetOuter1" result="shadowBlurOuter1"/>
      <feComposite in="shadowBlurOuter1" in2="SourceAlpha" operator="out" result="shadowBlurOuter1"/>
      <feColorMatrix values="0 0 0 0 0.99607843 0 0 0 0 0.7137255 0 0 0 0 0.19607843 0 0 0 0.3 0" in="shadowBlurOuter1"/>
    </filter>
    <filter x="-22.6%" y="-10.6%" width="145.1%" height="141.5%" filterUnits="objectBoundingBox" id="d">
      <feOffset dy="14" in="SourceAlpha" result="shadowOffsetOuter1"/>
      <feGaussianBlur stdDeviation="9" in="shadowOffsetOuter1" result="shadowBlurOuter1"/>
      <feColorMatrix values="0 0 0 0 0.99607843 0 0 0 0 0.7137255 0 0 0 0 0.19607843 0 0 0 0.3 0" in="shadowBlurOuter1"/>
    </filter>
    <path d="M57.3825547,16.6671252 C56.2117999,15.0843216 49.3786159,6.46906501 46.7680316,3.18193612 C46.1289184,2.37721435 45.1482311,1.90702871 44.1097632,1.90702871 L16.4094217,1.90702871 C15.3463819,1.90702871 14.3455018,2.39983998 13.7083957,3.23651915 L3.57807331,16.553163 C3.43508175,16.739642 3.32517714,16.9490072 3.2526778,17.1714057 C3.01620346,17.9032459 2.60699581,19.6304052 3.24100006,21.4614131 C3.32286592,21.6966363 3.44700278,21.9154376 3.60793671,22.1073384 L27.3182604,49.5139639 C28.0206281,50.3362544 29.1412659,51.1369619 30.1195811,51.1369619 C31.4243867,51.1369619 32.1751684,50.8106628 33.2596782,49.8283767 C35.0214972,48.2332176 56.6387066,22.6780361 57.4028083,21.7358923 C57.7178641,21.3458347 58.3608091,17.9894214 57.3825547,16.6671252 L57.3825547,16.6671252 Z" id="b"/>
    <path d="M23.2669778,14.4813425 L37.7482943,14.4813425 C39.3479021,14.4813425 40.6446184,15.7780414 40.6446184,17.3774755 C40.6446184,18.977118 39.3479021,20.2738169 37.7482943,20.2738169 L23.2669778,20.2738169 C21.6674308,20.2738169 20.3707145,18.977118 20.3707145,17.3774755 C20.3707145,15.7780414 21.6674308,14.4813425 23.2669778,14.4813425 Z" id="e"/>
    <linearGradient x1="50%" y1="100%" x2="50%" y2="0%" id="c">
      <stop stop-color="#FEB22B" offset="0%"/>
      <stop stop-color="#FAE37B" offset="100%"/>
    </linearGradient>
  </defs>
  <g fill="none" fill-rule="evenodd">
    <g transform="translate(9 .546)">
      <use fill="#000" filter="url(#a)" xlink:href="#b"/>
      <use fill="url(#c)" xlink:href="#b"/>
    </g>
    <g transform="translate(9 .546)">
      <use fill="#000" filter="url(#d)" xlink:href="#e"/>
      <use fill="#FFF" xlink:href="#e"/>
    </g>
    <text font-family="PingFang-SC-Medium, PingFang SC" font-size="24" font-weight="400" fill="#404040" transform="translate(-13)">
      <tspan x="28.5" y="104">积分</tspan>
    </text>
  </g>
</svg>
```
同样的图形使用webpack的svg-sprite-loader生成的图像可以正常加载。