Prevent overriding of Disposable.dispose

A common mistake made was overriding dispose and not calling super.dispose,
so a TS trick is now used to move it to a readonly property instead of a
member method which prevents it being overridden.

src/common/buffer/Marker.ts

@@ -3,35 +3,28 @@
  * @license MIT
  */
 
-import { EventEmitter, IEvent } from 'common/EventEmitter';
-import { Disposable } from 'common/Lifecycle';
+import { EventEmitter } from 'common/EventEmitter';
+import { Disposable, toDisposable } from 'common/Lifecycle';
 import { IMarker } from 'common/Types';
 
 export class Marker extends Disposable implements IMarker {
   private static _nextId = 1;
 
   private _id: number = Marker._nextId++;
-  public isDisposed: boolean = false;
-
   public get id(): number { return this._id; }
 
   private readonly _onDispose = new EventEmitter<void>();
   public readonly onDispose = this._onDispose.event;
 
+  public get isDisposed(): boolean { return this._isDisposed; };
+
   constructor(
     public line: number
   ) {
     super();
-  }
-
-  public dispose(): void {
-    if (this.isDisposed) {
-      return;
-    }
-    this.isDisposed = true;
-    this.line = -1;
-    // Emit before super.dispose such that dispose listeners get a change to react
-    this._onDispose.fire();
-    super.dispose();
+    this.register(toDisposable(() => {
+      this.line = -1;
+      this._onDispose.fire();
+    }));
   }
 }