feat(@angular-devkit/build-angular): add watch mode to server

[Toxicable]

Adds watch mode into server build

[tobi-or-not-tobi]

@Toxicable I got a notion of this PR on your talk at ng-air today - great job! I'm wondering if this is a generic solution, so it can be used for building other apps then universal as well based on file changes. I'm after a watch mode for libraries, see #730. Ideally, we can configure watch modes for specific apps/project.

[Toxicable]

@tobi-or-not-tobi No this wont help with your issue sorry.
This is for Universal only, since libraries undergo a much different build/bundle process it cannot be applied to that either.

[kondi]

Is it possible to have some explanation or info about "further discussion"? Would be good to have this option soon.

[filipesilva]

@kondi we discussed this last friday and I think this PR is good to get in as is.

[mgol]

It doesn't seem to be working correctly for me with @angular-devkit/build-angular 0.7.0-beta.1. Under my project's architect key I have the server target defined as follows:

"server": {
  "builder": "@angular-devkit/build-angular:server",
  "options": {
    "outputPath": "dist/server",
    "main": "src/main.server.ts",
    "tsConfig": "src/tsconfig.server.json",
    "watch": true
  }
}

There is no error when I run:

ng run phoenix:server

it compiles but it then exits immediately, not entering watch mode.

For comparison, watch works correctly for the browser part which has the following specified:

"builder": "@angular-devkit/build-angular:browser"

[mgol]

Comparing the code of the browser and server parts I see that the browser one doesn't complete the observable in watch mode:
https://github.com/angular/devkit/blob/v6.1.0-beta.1/packages/angular_devkit/build_angular/src/browser/index.ts#L111
while the server one has no special logic for watch here:
https://github.com/angular/devkit/blob/v6.1.0-beta.1/packages/angular_devkit/build_angular/src/server/index.ts#L84
Can that be a problem?

[Toxicable]

@mgol Yup it was the problem, just needed a simple check.
Should be fixed in the version that was just released