fix(@angular/cli): set correct mainFields for ng test when targeting …

[devoto13]

…es2015

Added test fails without the fix with the error described in the linked issue.

Fixes #8379

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[googlebot]

CLAs look good, thanks!

[devoto13]

It looks like tests failed because of angular/devkit@6d7a7ea#diff-39adc46a1150e1880c972fc429a66aa6. Will rebase once it is fixed in master.

[filipesilva]

LGTM, thanks for fixing this.

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