fix(ivy): ensure DebugNode/DebugElement are tree-shakeable in Ivy (#35003)

There are different `DebugNode`/`DebugElement` implementations (and
associated helper functions) for ViewEngine and Ivy. Additionally, these
classes/functions, which are defined inside the `core` package, are
imported by the `platform-browser` package.

Previously, this code was not tree-shaken as expected in Ivy. #30130
partially addressed the issue, but only for the case where `core` and
`platform-browser` end up in the same closure after webpack's scope
hoisting. In cases where this is not the case, our webpack/terser based
tooling is not capable of tree-shaking it.

This commit fixes the problem, by ensuring that the code retained in Ivy
mode (due to the cross-package import) does not unnecessarily reference
`DebugNode`/`DebugElement`, allowing the code to be tree-shaken away.
This results in a 7.6KB reduction in the size of the main angular.io
bundle.

Jira issue: [FW-1802](https://angular-team.atlassian.net/browse/FW-1802)

PR Close #35003

packages/core/src/core_private_export.ts

@@ -12,6 +12,7 @@ export {defaultIterableDiffers as ɵdefaultIterableDiffers, defaultKeyValueDiffe
 export {devModeEqual as ɵdevModeEqual, isListLikeIterable as ɵisListLikeIterable} from './change_detection/change_detection_util';
 export {ChangeDetectorStatus as ɵChangeDetectorStatus, isDefaultChangeDetectionStrategy as ɵisDefaultChangeDetectionStrategy} from './change_detection/constants';
 export {Console as ɵConsole} from './console';
+export {getDebugNodeR2 as ɵgetDebugNodeR2} from './debug/debug_node';
 export {inject, setCurrentInjector as ɵsetCurrentInjector, ɵɵinject} from './di/injector_compatibility';
 export {getInjectableDef as ɵgetInjectableDef, ɵɵInjectableDef, ɵɵInjectorDef} from './di/interface/defs';
 export {INJECTOR_SCOPE as ɵINJECTOR_SCOPE} from './di/scope';

packages/core/src/debug/debug_node.ts

@@ -727,6 +727,18 @@ export function getDebugNode__POST_R3__(nativeNode: any): DebugNode|null {
  */
 export const getDebugNode: (nativeNode: any) => DebugNode | null = getDebugNode__PRE_R3__;
 
+
+export function getDebugNodeR2__PRE_R3__(nativeNode: any): DebugNode|null {
+  return getDebugNode__PRE_R3__(nativeNode);
+}
+
+export function getDebugNodeR2__POST_R3__(_nativeNode: any): DebugNode|null {
+  return null;
+}
+
+export const getDebugNodeR2: (nativeNode: any) => DebugNode | null = getDebugNodeR2__PRE_R3__;
+
+
 export function getAllDebugNodes(): DebugNode[] {
   return Array.from(_nativeNodeToDebugNode.values());
 }