setSymbolColorExpression(expression) {
    this.hasSymbol_ = true;
    this.symbolColorExpression_ = expression;
    return this;
  }
Sets an expression to compute the color of the shape.
This expression can use all the uniforms, varyings and attributes available
in the fragment shader, and should evaluate to a `vec4` value.
@param {string} expression Color expression
@return {ShaderBuilder} the builder object