Don't require babel to be installed globally

[wincent]

While investigating an issue reported for the todomvc example
(#408), I tried to repro as follows:

git checkout master
git fetch upstream
git merge upstream/master
npm install && npm run update-schema && npm run start

But it went boom during the npm install because I didn't have babel
installed globally:

sh: babel: command not found

(Full output at https://gist.github.com/wincent/3221694ee3fdc46084f4)

What's happening here:

• example's preinstall tries to run install from the top-level
• top-level install tries to install babel-relay-plugin dependency
• babel-relay-plugin build step runs and tries to shell out to babel
• babel call will blow up if not globally installed, even though babel
  is listed as a dev dependency

Workaround:

npm install -g babel

Fix employed here: we add npm install --ignore-scripts to the
babel-relay-plugin build recipe, causing it to become available in
node_modules and therefore the $PATH when the babel command later
runs. Note that without the --ignore-scripts we get into an infinite
loop, although I am not sure of the exact chain of cause and effect
because all we see on the console at that point is this, endlessly
repeated:

> babel-relay-plugin@0.2.6 build /Users/glh/code/relay/scripts/babel-relay-plugin
> rm -rf lib; npm install; npm run babel

Also note the use of ; instead of && in the interest of preserving
potential future compatibility with Windows.

I think this change should be ok for end users or the
babel-relay-plugin, as they don't end up running the build script and
instead just get whatever we built into lib/.

Test Plan: Blow away global babel (npm uninstall -g babel) and example
node_modules and then npm install from inside example or top-level,
see it no longer blows from.

[wincent]

CC @yungsters who knows this stuff; I might be unaware of some constraints.

[yungsters]

Note that without the --ignore-scripts we get into an infinite loop, although I am not sure of the exact chain of cause and effect [...]

According to npm-build(1), the build script is a plumbing command called by npm install and npm link. So if the build script calls npm install, you have an infinite loop.

---

My understanding is that if you have a file://... dependency in your package.json (like our examples do), it won't install the dependency's dependencies. This is why babel-relay-plugin's dependency on babel is not installed, which is what causes the problem raised by this pull request.

If you take a look at the example's preinstall script, you'll notice it works around this by installing the root directory's dependencies:

"preinstall": "cd ../.. && npm install --ignore-scripts",

The --ignore-scripts flag means npm install should install the dependencies without running the build script. Why's that? Well, it appears that running npm install does run the build script of file://... dependencies. If we left out --ignore-scripts, we would end up building the react-relay package twice (which is not wrong, just unnecessarily time consuming).

NOTE:
In my opinion, npm's behavior around file://... dependencies is kind of messed up.

---

So back to the problem at hand — babel is a dependency of babel-relay-plugin, which is a file://... dependency of the examples. The reason that babel has an error when installing an example is that the babel-relay-plugin's build script is being executed without first installing its dependencies.

I don't think that changing babel-relay-plugin's build script is the right fix. This would be the equivalent of replacing the example's existing preinstall script by adding npm install --ignore-scripts to react-relay's build script, which is definitely wrong.

Instead, it seems that any package that has a file://... dependency needs to ensure that dependency's package dependencies are installed using npm install --ignore-scripts. So instead, we can just change the example's preinstall script to also correctly handle babel-relay-plugin:

"preinstall": "cd ../.. && npm install --ignore-scripts && cd scripts/babel-relay-plugin && npm install --ignore-scripts",

[wincent]

Whoops, I accidentally closed this. I'll fix it up and resubmit.

[wincent]

Ok, I think this should be good to go now.

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