code: function legalStyle(localStyle, legalSet) {
      //flatten array, easy for manipulating data
      let tmp = flattenArray(localStyle)
      let obj = {}
      for (let k in tmp) {
          //save legal declaration to output
          if (legalSet.has(k))
              obj[k] = tmp[k];
      }
      return obj;
  }
docs: constrain style,to remove illegal style warning,for example,
  setting fontSize for View may cause warning in debugger mode.
  @param localStyle style waiting for constraining.
  @param legalSet  specify legal style set for specify React Native element,eg. View or Text