Should Tree use OnPush?

[jelbourn]

Right now, CdkTree explicitly marks itself as ChangeDetectionStrategy.Default:

components/src/cdk/tree/tree.ts

Lines 65 to 69 in 5138328

	// The "OnPush" status for the `CdkTree` component is effectively a noop, so we are removing it.
	// The view for `CdkTree` consists entirely of templates declared in other views. As they are
	// declared elsewhere, they are checked when their declaration points are checked.
	// tslint:disable-next-line:validate-decorators
	changeDetection: ChangeDetectionStrategy.Default,

I suspect the rationale here is no longer may no longer be accurate. My understanding is that, under Ivy, change detection happens at both the declaration point (the end-developer's component) and the insertion point (the cdkTreeNodeOutlet inside the CdkTree template).

We should determine whether we should change the tree back to OnPush.

[crisbeto]

This was actually made specifically for Ivy, because of the different behavior for transplanted views.