[addon-docs] breaks if components feature dynamic title

[thebuilder]

Describe the bug
Using addon-docs with CSF components, that feature a dynamic title fails to load, throwing an error instead.

This happens if the title is using a function or a tagged template string.

Stacktrace

Module build failed (from ./node_modules/@storybook/source-loader/dist/server/index.js):
TypeError: Cannot read property 'toLowerCase' of undefined
  at sanitize (./node_modules/@storybook/router/dist/utils.js:64:17)
  at sanitizeSafe (./node_modules/@storybook/router/dist/utils.js:72:19)
  at toId (./node_modules/@storybook/router/dist/utils.js:82:20)
  at handleExportedName (./node_modules/@storybook/source-loader/dist/server/abstract-syntax-tree/parse-helpers.js:87:6)
  at Controller.enter (./node_modules/@storybook/source-loader/dist/server/abstract-syntax-tree/traverse-helpers.js:109:60)
  at Controller.__execute (./node_modules/estraverse/estraverse.js:330:31)
  at Controller.traverse (./node_modules/estraverse/estraverse.js:434:28)
  at Object.traverse (./node_modules/estraverse/estraverse.js:646:27)
  at findExportsMap (./node_modules/@storybook/source-loader/dist/server/abstract-syntax-tree/traverse-helpers.js:101:14)
  at generateStoriesLocationsMap (./node_modules/@storybook/source-loader/dist/server/abstract-syntax-tree/generate-helpers.js:138:63)
  at inject (./node_modules/@storybook/source-loader/dist/server/abstract-syntax-tree/inject-decorator.js:74:56)
  at readAsObject (./node_modules/@storybook/source-loader/dist/server/dependencies-lookup/readAsObject.js:45:47)
  at readStory (./node_modules/@storybook/source-loader/dist/server/dependencies-lookup/readAsObject.js:100:10)
  at Object.transform (./node_modules/@storybook/source-loader/dist/server/build.js:15:38)

Expected behavior
Docs should be able to load files with dynamic title

Code snippets

import * as React from 'react'
import base from 'paths.macro'

export default {
  title: `Modules|${base}`
}

export const example = () => <div />

or

import * as React from 'react'
import base from 'paths.macro'

export default {
  title: `${base}` // Template strings causes the same issue
}

export const example = () => <div />

System:

Environment Info:

  System:
    OS: macOS 10.14.6
    CPU: (12) x64 Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz
  Binaries:
    Node: 12.9.1 - ~/.config/nvm/12.9.1/bin/node
    Yarn: 1.17.3 - /usr/local/bin/yarn
    npm: 6.10.2 - ~/.config/nvm/12.9.1/bin/npm
  Browsers:
    Chrome: 76.0.3809.132
    Firefox: 68.0.2
    Safari: 12.1.2
  npmPackages:
    @storybook/addon-a11y: ^5.2.0-rc.0 => 5.2.0-rc.0 
    @storybook/addon-actions: ^5.2.0-rc.0 => 5.2.0-rc.0 
    @storybook/addon-docs: ^5.2.0-rc.0 => 5.2.0-rc.0 
    @storybook/addon-links: ^5.2.0-rc.0 => 5.2.0-rc.0 
    @storybook/addon-viewport: ^5.2.0-rc.0 => 5.2.0-rc.0 
    @storybook/addons: ^5.2.0-rc.0 => 5.2.0-rc.0 
    @storybook/react: ^5.2.0-rc.0 => 5.2.0-rc.0 
    @storybook/source-loader: ^5.2.0-rc.0 => 5.2.0-rc.0 
    @storybook/theming: ^5.2.0-rc.0 => 5.2.0-rc.0 

Additional context
This is based on the documented approach to generating path names use babel macros:
https://github.com/storybookjs/storybook/blob/next/docs/src/pages/basics/writing-stories/index.md#generating-nesting-path-based-on-__dirname

[thebuilder]

Is it possible for the source-loader to lazy evaluate the title?

[stale]

Hi everyone! Seems like there hasn't been much going on in this issue lately. If there are still questions, comments, or bugs, please feel free to continue the discussion. Unfortunately, we don't have time to get to every issue. We are always open to contributions so please send us a pull request if you would like to help. Inactive issues will be closed after 30 days. Thanks!

[stale]

Hi everyone! Seems like there hasn't been much going on in this issue lately. If there are still questions, comments, or bugs, please feel free to continue the discussion. Unfortunately, we don't have time to get to every issue. We are always open to contributions so please send us a pull request if you would like to help. Inactive issues will be closed after 30 days. Thanks!

[stale]

Hi everyone! Seems like there hasn't been much going on in this issue lately. If there are still questions, comments, or bugs, please feel free to continue the discussion. Unfortunately, we don't have time to get to every issue. We are always open to contributions so please send us a pull request if you would like to help. Inactive issues will be closed after 30 days. Thanks!

[shilman]

Yippee!! I just released https://github.com/storybookjs/storybook/releases/tag/v5.3.0-beta.14 containing PR #8995 that references this issue. Upgrade today to try it out!

You can find this prerelease on the @next NPM tag.

Closing this issue. Please re-open if you think there's still more to do.