allow project using schematics to save in devDeps instead of deps in package.json

[timstoddard]

Bug Report or Feature Request (mark with an x)

- [ ] bug report -> please search issues before submitting
- [x] feature request

Command (mark with an x)

- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [x] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

Versions

Node: 10.8.0
Npm: 6.4.0

Angular CLI: 6.1.3
Node: 10.8.0
OS: win32 x64
Angular: 6.1.2
... common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic

Package                      Version
------------------------------------------------------
@angular-devkit/architect    0.7.3 (cli-only)
@angular-devkit/core         0.7.4
@angular-devkit/schematics   0.7.4
@schematics/angular          0.7.4
@schematics/update           0.7.3 (cli-only)
rxjs                         6.2.2
typescript                   2.9.2

Windows 10

Repro steps

n/a

The log given by the failure

n/a

Desired functionality

As a package author who has implemented schematics into our angular add-on, it would be great to have control over whether our package gets saved in dependencies or devDependencies - our add-on is intended for use in the dev workflow, but not in production. It seems that this functionality has been somewhat implemented but is not available for use yet. Will this be made available for developers to access at some point?

Mention any other details that might be useful

https://github.com/angular/angular-cli/blob/master/packages/angular/cli/tasks/npm-install.ts
^^ The npm install task function in that file takes in a save boolean argument, but when the function is called here there is no matching parameter passed into it, so it always uses the default value of true.

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