function expectTransform(script) {
    const {code, warnings} = transformer.run(script);

    return {
      toReturn(expectedValue) {
        expect(code).to.equal(expectedValue);
        return this;
      },
      withWarnings(expectedWarnings) {
        expect(warnings).to.deep.equal(expectedWarnings);
        return this;
      },
      withoutWarnings() {
        return this.withWarnings([]);
      },
    };
  }
Generates functions that are used in all transform-tests.
@param  {String[]} transformNames Config for Transformer class
@return {Object} functions expectTransform() and expectNoChange()