fix(service-worker): esm2015 points to wrong path

[Doehl]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

The Service Worker package points to a non-existing path for the esm2015 build.
Current value is "esm15"

This makes build with Google Closure Compiler impossible with following settings.
--package_json_entry_names es2015
--process_common_js_modules

Issue Number: N/A

What is the new behavior?

The esm2015 value should be "esm2015".

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

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