Add return types to clone()

packages/cells/src/widget.ts

@@ -314,7 +314,7 @@ class Cell extends Widget {
   /**
    * Clone the cell, using the same model.
    */
-  clone() {
+  clone(): Cell {
     let constructor = this.constructor as typeof Cell;
     return new constructor({
       model: this.model,
@@ -623,7 +623,7 @@ class CodeCell extends Cell {
   /**
    * Clone the cell, using the same model.
    */
-  clone() {
+  clone(): CodeCell {
     let constructor = this.constructor as typeof CodeCell;
     return new constructor({
       model: this.model,
@@ -875,7 +875,7 @@ class MarkdownCell extends Cell {
   /**
    * Clone the cell, using the same model.
    */
-  clone() {
+  clone(): MarkdownCell {
     let constructor = this.constructor as typeof MarkdownCell;
     return new constructor({
       model: this.model,