[bug report] server exit when saving src/pages/index.js for a couple of times

[casprwang]

This isn't a big issue, but I found kept quickly saving the src/pages/index.js for 5-6 times can cause gatsby server stop running.

log:

changed file at .../src/pages/index.js
TypeError: Cannot read property 'size' of undefined
    at .../node_modules/gatsby-source-filesystem/gatsby-node.js:50:20
    at FSReqWrap.oncomplete (fs.js:114:15)

    "gatsby": "1.0.0-alpha21",
    "gatsby-link": "1.0.0-alpha20",
    "gatsby-plugin-postcss-sass": "1.0.0-alpha.22",
    "gatsby-plugin-sharp": "1.0.0-alpha19",
    "gatsby-remark-copy-linked-files": "1.0.0-alpha17",
    "gatsby-remark-responsive-iframe": "1.0.0-alpha19",
    "gatsby-remark-responsive-image": "1.0.0-alpha20",
    "gatsby-source-filesystem": "1.0.0-alpha19-alpha.a2f3e881",
    "gatsby-transformer-remark": "1.0.0-alpha20",

Mac OS Node 8

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