fix(@angular/cli): don't fail install due to node-sass

[filipesilva]

This change should reduce the amount of problems on npm installs.

It makes the node-sass dependency optional, and so install will not fail when it cannot be installed.

For projects that do not use sass, there is no side-effect of an failed node-sass install.

For projects that do use node-sass and the install failed, they should see the following self-explanatory error message:

ERROR in ./src/app/app.component.scss
Module build failed: Error: Missing binding D:\sandbox\master-project\node_modules\node-sass\vendor\win32-x64-51\binding.node
Node Sass could not find a binding for your current environment: Windows 64-bit with Node.js 7.x

Found bindings for the following environments:

This usually happens because your environment has changed since running `npm install`.
Run `npm rebuild node-sass` to build the binding for your current environment.
    at module.exports (D:\sandbox\master-project\node_modules\node-sass\lib\binding.js:15:13)
    at Object.<anonymous> (D:\sandbox\master-project\node_modules\node-sass\lib\index.js:14:35)

Fix #4429

[hansl]

Blocking this for now.

[hansl]

LGTM

[hansl]

I wish there was a way to test this though.

[slashbrackets]

filipesilva commented on Mar 7 above that the fix was to make node-sass optional which allows the install to continue without failure. However, if a project needs to use node-sass then they get the error that it's missing. What he didn't include is how do you get it if you're behind a firewall that blocks the node-sass binaries?

[filipesilva]

@slashbrackets in that case you can add some global variables that tell node-sass where to download the binary from: https://github.com/sass/node-sass#binary-configuration-parameters

You can also setup your system to compile node-sass yourself.

If you cannot do either, then I don't think node-sass can be used at all in your environment.

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