Ensure fs.Stats (and other classes on fs) are not mutated.

[rwjblue]

fs.Stats is a standard prototypal class (e.g. function Stats() {}), so it was being wrapped by our fs monitoring system. Unfortunately, wrapping fs.Stats in a plain function (which did return original.apply(this, arguments)) doesn't properly emulate a number of "standard" class behaviors. Specifically:

• static methods and properties are lost
• accessing fs.Stats.prototype does not refer to the original prototype

This last item was causing the following error when using esm:

TypeError: Function.prototype.apply was called on undefined, which is a undefined and not a function
    at $o (/home/travis/build/ember-cli/ember-cli/tmp/node-FGpJBujF.tmp/node_modules/esm/esm.js:1:224377)
    at xu (/home/travis/build/ember-cli/ember-cli/tmp/node-FGpJBujF.tmp/node_modules/esm/esm.js:1:226413)
    at wu (/home/travis/build/ember-cli/ember-cli/tmp/node-FGpJBujF.tmp/node_modules/esm/esm.js:1:227391)
    at Eu (/home/travis/build/ember-cli/ember-cli/tmp/node-FGpJBujF.tmp/node_modules/esm/esm.js:1:227999)
    at Module.<anonymous> (/home/travis/build/ember-cli/ember-cli/tmp/node-FGpJBujF.tmp/node_modules/esm/esm.js:1:295976)
    at n (/home/travis/build/ember-cli/ember-cli/tmp/node-FGpJBujF.tmp/node_modules/esm/esm.js:1:279589)
    at getFeatures (/home/travis/build/ember-cli/ember-cli/tmp/node-FGpJBujF.tmp/node_modules/@ember-data/-build-infra/src/features.js:5:33)
    at Object.<anonymous> (/home/travis/build/ember-cli/ember-cli/tmp/node-FGpJBujF.tmp/node_modules/@ember-data/-build-infra/src/features.js:31:18)

This is because esm is essentially doing:

let isFile = fs.Stats.prototype.isFile;

To stash off the isFile implementation for recurring usage. The fact that when wrapped fs.Stat.prototype.isFile was undefined caused the error.

[hjdivad]