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

`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:

```js
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.

index.js

@@ -13,7 +13,15 @@ let hasActiveInstance = false;
 class FSMonitor {
   constructor() {
     this.state = 'idle';
-    this.blacklist = ['createReadStream', 'createWriteStream', 'ReadStream', 'WriteStream'];
+    this.blacklist = [
+      'createReadStream',
+      'createWriteStream',
+      'Dirent',
+      'FSWatcher',
+      'ReadStream',
+      'Stats',
+      'WriteStream'
+    ];
   }
 
   start() {

tests.js

@@ -3,7 +3,8 @@
 const FSMonitor = require('./');
 const expect = require('chai').expect;
 const fs = require('fs');
-const originalStatSync = fs.statSync;
+
+const originalFS = Object.assign({}, fs);
 
 describe('FSMonitor', function() {
   it('will only allow one active instance at a time', function() {
@@ -28,17 +29,65 @@ describe('FSMonitor', function() {
     monitor0.stop();
   });
 
+  it('does not mutate the prototype of classes on fs [GH#22]', function() {
+    let monitor = new FSMonitor();
+
+    expect(typeof fs.Stats.prototype.isFile).to.equal('function');
+
+    monitor.start();
+
+    expect(typeof fs.Stats.prototype.isFile).to.equal('function', 'after updating fs');
+
+    monitor.stop();
+
+    expect(typeof fs.Stats.prototype.isFile).to.equal('function');
+  });
+
+  it('avoids mutating known classes on `fs` [GH#22]', function() {
+    let monitor = new FSMonitor();
+
+    expect(fs.Stats.prototype.isFile).to.be;
+    expect(fs.Stats).to.equal(originalFS.Stats);
+    expect(fs.Dirent).to.equal(originalFS.Dirent);
+    expect(fs.FSWatcher).to.equal(originalFS.FSWatcher);
+    expect(fs.FileHandle).to.equal(originalFS.FileHandle);
+    expect(fs.ReadStream).to.equal(originalFS.ReadStream);
+    expect(fs.WriteStream).to.equal(originalFS.WriteStream);
+
+    try {
+      monitor.start();
+
+      // should not have been changed
+      expect(fs.Stats).to.equal(originalFS.Stats);
+      expect(fs.Dirent).to.equal(originalFS.Dirent);
+      expect(fs.FSWatcher).to.equal(originalFS.FSWatcher);
+      expect(fs.FileHandle).to.equal(originalFS.FileHandle);
+      expect(fs.ReadStream).to.equal(originalFS.ReadStream);
+      expect(fs.WriteStream).to.equal(originalFS.WriteStream);
+    } finally {
+      // ensure we stop and detach even if we fail an assertion
+      monitor.stop();
+    }
+
+    expect(fs.Stats).to.equal(originalFS.Stats);
+    expect(fs.Dirent).to.equal(originalFS.Dirent);
+    expect(fs.FSWatcher).to.equal(originalFS.FSWatcher);
+    expect(fs.FileHandle).to.equal(originalFS.FileHandle);
+    expect(fs.ReadStream).to.equal(originalFS.ReadStream);
+    expect(fs.WriteStream).to.equal(originalFS.WriteStream);
+  });
+
   describe('.prototype.stop', function() {
     it('restores fs functions to their defaults', function() {
       let monitor = new FSMonitor();
 
-      expect(fs.statSync).to.equal(originalStatSync);
+      expect(fs.statSync).to.equal(originalFS.statSync);
 
       monitor.start();
-      expect(fs.statSync).to.not.equal(originalStatSync);
+      expect(fs.statSync).to.not.equal(originalFS.statSync);
 
       monitor.stop();
-      expect(fs.statSync).to.equal(originalStatSync);
+      expect(fs.statSync).to.equal(originalFS.statSync);
     });
   });
 });