code: function getFunctionFromDefaultAndShaderValue(sceneDrawGroup, ccssProperty, defaultValue, shaderValue) {
      if (referenceCSS[ccssProperty].type === 'color') {
          defaultValue = `'${color.normalize(defaultValue, tangramReference)}'`;
      }
      var fn = `var _value=${defaultValue};`;
      shaderValue.js.forEach(function (code) {
          if (code.search(/data\['mapnik::\S+'\]/) >= 0) {
              throw new Error('mapnik selector present in the CartoCSS');
          }
          fn += code;
      });
      if (referenceCSS[ccssProperty].type === 'color') {
          fn += getColorOverrideCode(sceneDrawGroup, ccssProperty.indexOf('fill') >= 0);
      }
      if (ccssProperty === 'line-width') {
          fn += '_value=_value*$meters_per_pixel;';
      }
      fn += 'return _value;';
      return wrapFn(fn);
  }
docs: Returns a function string that sets the value to the default one and then executes the shader value code