Fix strict null checks in ./common

Part of #1319

src/common/CircularList.ts

@@ -11,7 +11,7 @@ import { ICircularList } from './Types';
  * overriding values at the start of the list.
  */
 export class CircularList<T> extends EventEmitter implements ICircularList<T> {
-  protected _array: T[];
+  protected _array: (T | undefined)[];
   private _startIndex: number;
   private _length: number;
 
@@ -36,7 +36,7 @@ export class CircularList<T> extends EventEmitter implements ICircularList<T> {
 
     // Reconstruct array, starting at index 0. Only transfer values from the
     // indexes 0 to length.
-    const newArray = new Array<T>(newMaxLength);
+    const newArray = new Array<T | undefined>(newMaxLength);
     for (let i = 0; i < Math.min(newMaxLength, this.length); i++) {
       newArray[i] = this._array[this._getCyclicIndex(i)];
     }
@@ -66,7 +66,7 @@ export class CircularList<T> extends EventEmitter implements ICircularList<T> {
    * @param index The index of the value to get.
    * @return The value corresponding to the index.
    */
-  public get(index: number): T {
+  public get(index: number): T | undefined {
     return this._array[this._getCyclicIndex(index)];
   }
 
@@ -78,7 +78,7 @@ export class CircularList<T> extends EventEmitter implements ICircularList<T> {
    * @param index The index to set.
    * @param value The value to set.
    */
-  public set(index: number, value: T): void {
+  public set(index: number, value: T | undefined): void {
     this._array[this._getCyclicIndex(index)] = value;
   }
 
@@ -104,7 +104,7 @@ export class CircularList<T> extends EventEmitter implements ICircularList<T> {
    * Removes and returns the last value on the list.
    * @return The popped value.
    */
-  public pop(): T {
+  public pop(): T | undefined {
     return this._array[this._getCyclicIndex(this._length-- - 1)];
   }
 

src/common/EventEmitter.ts

@@ -30,14 +30,15 @@ export class EventEmitter extends Disposable implements IEventEmitter, IDisposab
   public addDisposableListener(type: string, handler: XtermListener): IDisposable {
     // TODO: Rename addDisposableEventListener to more easily disambiguate from Dom listener
     this.on(type, handler);
+    let disposed = false;
     return {
       dispose: () => {
-        if (!handler) {
+        if (disposed) {
           // Already disposed
           return;
         }
         this.off(type, handler);
-        handler = null;
+        disposed = true;
       }
     };
   }

src/common/Types.ts

@@ -25,10 +25,10 @@ export interface ICircularList<T> extends IEventEmitter {
   length: number;
   maxLength: number;
 
-  get(index: number): T;
+  get(index: number): T | undefined;
   set(index: number, value: T): void;
   push(value: T): void;
-  pop(): T;
+  pop(): T | undefined;
   splice(start: number, deleteCount: number, ...items: T[]): void;
   trimStart(count: number): void;
   shiftElements(start: number, count: number, offset: number): void;