Move _isDisposed to Disposable class

src/Terminal.ts

@@ -193,7 +193,6 @@ export class Terminal extends EventEmitter implements ITerminal, IDisposable, II
 
   // Store if user went browsing history in scrollback
   private _userScrolling: boolean;
-  private _isDisposed: boolean = false;
 
   private _inputHandler: InputHandler;
   public soundManager: SoundManager;
@@ -234,7 +233,6 @@ export class Terminal extends EventEmitter implements ITerminal, IDisposable, II
   }
 
   public dispose(): void {
-    this._isDisposed = true;
     super.dispose();
     this._customKeyEventHandler = null;
     removeTerminalFromCache(this);

src/common/Lifecycle.ts

@@ -11,6 +11,7 @@ import { IDisposable } from 'xterm';
  */
 export abstract class Disposable implements IDisposable {
   protected _disposables: IDisposable[] = [];
+  protected _isDisposed: boolean = false;
 
   constructor() {
   }
@@ -19,6 +20,7 @@ export abstract class Disposable implements IDisposable {
    * Disposes the object, triggering the `dispose` method on all registered IDisposables.
    */
   public dispose(): void {
+    this._isDisposed = true;
     this._disposables.forEach(d => d.dispose());
     this._disposables.length = 0;
   }