feat(@angular/cli): improve common bundling performance

[clydin]

This adds an asynchronously loaded common chunk.
For a medium-sized project, this reduced aggregate bundle size by ~10%.

Partially addresses #6204

[filipesilva]

@clydin it seems like the upcoming Webpack scope hoisting would fully address the issue.

It would be better to wait for that rather than implement an incomplete solution. What do you think?

[filipesilva]

Turns out scope hoisting has absolutely no relevant to this PR. It LGTM but is missing a test, can you add one please?

[nathanmarks]

@clydin are you still working on this?

[clydin]

Yes. Barring any issues, release goal is for 1.2.

[nathanmarks]

@clydin Since 1.2-beta.1 is already out, and this is still missing a test requested by @filipesilva , is it still going to make it?

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