refactor(core): change abstract classes for interfaces (angular#12324)

BREAKING CHANGE: Because all lifecycle hooks are now interfaces
the code that uses 'extends' keyword will no longer compile.

To migrate the code follow the example below:

Before:
```
@component()
class SomeComponent extends OnInit {}
```
After:
```
@component()
class SomeComponent implements OnInit {}
```

we don't expect anyone to be affected by this change.

Closes angular#10083

modules/@angular/core/src/metadata/lifecycle_hooks.ts

@@ -49,7 +49,7 @@ export const LIFECYCLE_HOOKS_VALUES = [
  *
  * @stable
  */
-export abstract class OnChanges { abstract ngOnChanges(changes: SimpleChanges): void; }
+export interface OnChanges { ngOnChanges(changes: SimpleChanges): void; }
 
 /**
  * @whatItDoes Lifecycle hook that is called after data-bound properties of a directive are
@@ -66,7 +66,7 @@ export abstract class OnChanges { abstract ngOnChanges(changes: SimpleChanges):
  *
  * @stable
  */
-export abstract class OnInit { abstract ngOnInit(): void; }
+export interface OnInit { ngOnInit(): void; }
 
 /**
  * @whatItDoes Lifecycle hook that is called when Angular dirty checks a directive.
@@ -89,7 +89,7 @@ export abstract class OnInit { abstract ngOnInit(): void; }
  *
  * @stable
  */
-export abstract class DoCheck { abstract ngDoCheck(): void; }
+export interface DoCheck { ngDoCheck(): void; }
 
 /**
  * @whatItDoes Lifecycle hook that is called when a directive, pipe or service is destroyed.
@@ -104,7 +104,7 @@ export abstract class DoCheck { abstract ngDoCheck(): void; }
  *
  * @stable
  */
-export abstract class OnDestroy { abstract ngOnDestroy(): void; }
+export interface OnDestroy { ngOnDestroy(): void; }
 
 /**
  *
@@ -118,7 +118,7 @@ export abstract class OnDestroy { abstract ngOnDestroy(): void; }
  *
  * @stable
  */
-export abstract class AfterContentInit { abstract ngAfterContentInit(): void; }
+export interface AfterContentInit { ngAfterContentInit(): void; }
 
 /**
  * @whatItDoes Lifecycle hook that is called after every check of a directive's content.
@@ -130,7 +130,7 @@ export abstract class AfterContentInit { abstract ngAfterContentInit(): void; }
  *
  * @stable
  */
-export abstract class AfterContentChecked { abstract ngAfterContentChecked(): void; }
+export interface AfterContentChecked { ngAfterContentChecked(): void; }
 
 /**
  * @whatItDoes Lifecycle hook that is called after a component's view has been fully
@@ -143,7 +143,7 @@ export abstract class AfterContentChecked { abstract ngAfterContentChecked(): vo
  *
  * @stable
  */
-export abstract class AfterViewInit { abstract ngAfterViewInit(): void; }
+export interface AfterViewInit { ngAfterViewInit(): void; }
 
 /**
  * @whatItDoes Lifecycle hook that is called after every check of a component's view.
@@ -155,4 +155,4 @@ export abstract class AfterViewInit { abstract ngAfterViewInit(): void; }
  *
  * @stable
  */
-export abstract class AfterViewChecked { abstract ngAfterViewChecked(): void; }
+export interface AfterViewChecked { ngAfterViewChecked(): void; }

tools/public_api_guard/core/typings/core.d.ts

@@ -1,21 +1,21 @@
 /** @stable */
-export declare abstract class AfterContentChecked {
-    abstract ngAfterContentChecked(): void;
+export interface AfterContentChecked {
+    ngAfterContentChecked(): void;
 }
 
 /** @stable */
-export declare abstract class AfterContentInit {
-    abstract ngAfterContentInit(): void;
+export interface AfterContentInit {
+    ngAfterContentInit(): void;
 }
 
 /** @stable */
-export declare abstract class AfterViewChecked {
-    abstract ngAfterViewChecked(): void;
+export interface AfterViewChecked {
+    ngAfterViewChecked(): void;
 }
 
 /** @stable */
-export declare abstract class AfterViewInit {
-    abstract ngAfterViewInit(): void;
+export interface AfterViewInit {
+    ngAfterViewInit(): void;
 }
 
 /** @experimental */
@@ -385,8 +385,8 @@ export interface DirectiveDecorator {
 }
 
 /** @stable */
-export declare abstract class DoCheck {
-    abstract ngDoCheck(): void;
+export interface DoCheck {
+    ngDoCheck(): void;
 }
 
 /** @stable */
@@ -678,18 +678,18 @@ export declare class NgZone {
 export declare const NO_ERRORS_SCHEMA: SchemaMetadata;
 
 /** @stable */
-export declare abstract class OnChanges {
-    abstract ngOnChanges(changes: SimpleChanges): void;
+export interface OnChanges {
+    ngOnChanges(changes: SimpleChanges): void;
 }
 
 /** @stable */
-export declare abstract class OnDestroy {
-    abstract ngOnDestroy(): void;
+export interface OnDestroy {
+    ngOnDestroy(): void;
 }
 
 /** @stable */
-export declare abstract class OnInit {
-    abstract ngOnInit(): void;
+export interface OnInit {
+    ngOnInit(): void;
 }
 
 /** @deprecated */