refactor(upgrade): unify spec code

[chrisse27]

Please check if the PR fulfills these 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)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

Tests in upgrade_spec.ts are mostly not making any use of "let" or "const" as well as arrow functions.

What is the new behavior?

This PR intends to unify the usage of "let", "const, and arrow functions across all tests in upgrade_spec.ts.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

• replace all variable declarations using 'var' by 'const' or 'let'
• replace es5 function declaration by arrow function where applicable

[chrisse27]

@vicb As suggested by you in PR #11411 here is the first attempt to unify the upgrade_spec.

@mhevery actually started using decorators in 6fc4652. I could also refactor the other tests using decorators instead of function calls (separate PR). What's your opinion on this?

[vicb]

Thanks.
+1 for refactoring

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