REF: convert PlaceholderWallet to typescript

class/wallets/placeholder-wallet.ts

@@ -4,34 +4,38 @@ export class PlaceholderWallet extends AbstractWallet {
   static type = 'placeholder';
   static typeReadable = 'Placeholder';
 
+  _isFailure: boolean;
+
   constructor() {
     super();
     this._isFailure = false;
   }
 
-  setSecret(newSecret) {
+  setSecret(newSecret: string): this {
     // so TRY AGAIN when something goes wrong during import has more consistent prefilled text
     this.secret = newSecret;
+
+    return this;
   }
 
-  allowSend() {
+  allowSend(): boolean {
     return false;
   }
 
-  getLabel() {
+  getLabel(): string {
     // no longer used in wallets carousel
     return this.getIsFailure() ? 'Wallet Import' : 'Importing Wallet...';
   }
 
-  allowReceive() {
+  allowReceive(): boolean {
     return false;
   }
 
-  getIsFailure() {
+  getIsFailure(): boolean {
     return this._isFailure;
   }
 
-  setIsFailure(value) {
+  setIsFailure(value: boolean): void {
     this._isFailure = value;
   }
 }