Previously, we had to check whether `NgElementStrategy` had been
instantiated before accessing it. This was tedious and error prone,
since it was easy to forget to add the check in new call sites.

This commit switches to using a getter, so that the check has to be
performed in one place and is transparent to call sites (including any
future ones).

PR Close #36114

- Remove imports that are not used.
- Remove private class properties that are not used outside of the
  constructor.

PR Close #36114

Previously, helper modules/components classes were declared even if the
tests were not run (because the environment did not support Custom
Elements for example).

This commit moves the declaration of the helpers inside the `describe()`
block, so they are not declared unnecessarily. This is in preparation of
adding more helpers that need to access variables declared inside the
`describe()` block.

PR Close #36114

Previously, the `TestStrategy` `NgElementStrategy` used in
`createCustomElement()` tests was created once and re-used in each test
(due to complications related to how `customElements.register()` works).
As a result, the `TestStrategy` instance's state (e.g. inputs) could be
polluted from previous tests and affect subsequent ones.

This commit ensures the strategy instance is reset before each test.

PR Close #36114

…ent (#36114)

Previously, if an element started out as a regular `HTMLElement` (not a
Custom Element) and was later upgraded to a Custom Element, any
properties corresponding to component inputs that were set on the
element before upgrading it would not be captured correctly and thus not
reflected on the instantiated component.

This commit fixes it by ensuring that such properties are captured
correctly.

Jira issue: [FW-2006](https://angular-team.atlassian.net/browse/FW-2006)

Fixes #30848
Closes #31416

PR Close #36114

…t is not called (#36114)

Previously, the correct behavior of Angular custom elements relied on
the constructor being called (and thus the `injector` property being
initialized). However, some polyfills (e.g. `document-register-element`)
do not call the constructor of custom elements, which resulted in the
`injector` property being undefined and the `NgElementStrategy` failing
to be instantiated.

This commit fixes it by being tolerant to the `injector` property being
undefined and falling back to the injector passed to the
`createCustomElement()` config.

NOTE:
We don't have proper tests exercising the situation where the
constructor is not called. For now this is tested using a Google
internal test suite (which is how this issue was caught).
This commit also adds a rudimentary unit test to emulate this situation.

PR Close #36114

…necting the element (#36114)

`createCustomElements()` creates some getters/setters for properties
corresponding to component inputs that delegate to the
`NgElementStrategy`. However, it is not guaranteed that the element's
`NgElementStrategy` will have been created when these getters/setters
are called, because some polyfills (e.g. `document-register-element`) do
not call the constructor.

Previously, trying to get/set input properties before connecting the
element to the DOM (via `connectedCallback()`) would fail due to
`NgElementStrategy` not being created.

This commit ensures that the `NgElementStrategy` is always created
before used inside the input property getters/setters (similar to how it
is done for other methods of `NgElement`).

Mentioned in https://github.com/angular/angular/pull/31416/files#r300326698.

PR Close #36114

Previously, we had to check whether `NgElementStrategy` had been
instantiated before accessing it. This was tedious and error prone,
since it was easy to forget to add the check in new call sites.

This commit switches to using a getter, so that the check has to be
performed in one place and is transparent to call sites (including any
future ones).

PR Close #36114

- Remove imports that are not used.
- Remove private class properties that are not used outside of the
  constructor.

PR Close #36114

Previously, helper modules/components classes were declared even if the
tests were not run (because the environment did not support Custom
Elements for example).

This commit moves the declaration of the helpers inside the `describe()`
block, so they are not declared unnecessarily. This is in preparation of
adding more helpers that need to access variables declared inside the
`describe()` block.

PR Close #36114