fix(@angular-devkit/build-angular): downlevel class properties when targeting Safari <=v15

[devversion]

The Angular compiler is dependent on static fields being attached to
user-defined classes. e.g. static ecmp = defineComponent.

These static fields sometimes rely on variables from outside of the
class. e.g. the Angular compiler generates constants for content
projection that are then accessed in the static field initializer.

Surprisingly such access to these variables may break in Safari <=v15
when a page is loaded without devtools open. The bug (already solved in
v16 of Safari)- is very subtle, hard to re-reproduce but basically
variable scope tracking is broken. This bug is triggered by additional
parenthesis in the initializer expression. See:
https://bugs.webkit.org/show_bug.cgi?id=236843.

The TypeScript compiler may generate such additional parenthesis when
it tries to adjust the this context when invoking methods, such as for
defining animations in the ecmp definition.

More details can be found here:
#24355 (comment)

To ensure Angular applications are not subject to this bug when
targeting Safari <=v15. v15 Safari, both for iOS and Mac is still part of
the default CLI browserslist with last 2 Safari majors (at time of
writing).

Note that it is important that the Babel plugin properly handles the
downleveling of static block-defined members. TypeScript will transform
static fields, like static ecmp into static { this.ecmp = X } when
useDefineForClassFields = false (which is the case for CLI apps). The
class properties plugin from Babel seems to handle this in an acceptable
way. Unlike actual static fields, Babel will not use helpers like
defineProperty for such extracted static blocks though. e.g.

See repro: https://gist.github.com/devversion/dec0dea26e348c509921bf62079b60be

class Test {
  x = true;

  static b = true;
  static {
    this.a = true;
  }
}

// into

class X {
  constructor() {
    _defineProperty(this, "x", true);
  }
}
_defineProperty(X, "b", true);
X.a = true;

note that in practice TypeScript with useDefineForClassFields = false
will put non-static members into the constructor as normal assignments
regardless- so there would be no change by the Babel plugin.

Fixes #24355

[devversion]

FYI: Discussed with @clydin that we should just apply for v14 too. The range can be that loose just to be safe. older versions will have the plugin enabled anyway.

[devversion]

This is a sanity test. Obviously it's not validating using AST, nor validating Safari. Should be helpful regardless though.

[clydin]

LGTM.
Just two suggestions:

• Can you add an additional test case for an explicit safari <=15 browserslist?
• Also can you add a comment that the default test case (no explicit browserslist) will need to be remove once Safari 17 is released to prevent this from failing?

[devversion]

@clydin Updated.

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