TypedEventEmitter: update emit fuction signature

Summary:
Somehow I cannot typecheck progress data with the union type:

    const onProgress = (progress: OperationProgress) => {
      // progressEmitter is TypedEventEmitter<'progress', OperationProgress>
      this.progressEmitter.emit('progress', progress);
                                ^^^^^^^^^^^^^^^^^^^^
    };

Error:

    Argument of type '["progress", OperationProgress]' is not assignable to parameter of type '[event: "error", data: Error] | [event: "progress", data: { id: string; kind: "queue"; queue: string[]; }] | [event: "progress", data: { id: string; kind: "spawn"; queue: string[]; }] | [event: "progress", data: { id: string; kind: "stderr"; message: string; }] | ... 5 more ... | [event: ...]'.
    Type '["progress", OperationProgress]' is not assignable to type '[event: "progress", data: { id: string; kind: "queue"; queue: string[]; }] | [event: "progress", data: { id: string; kind: "spawn"; queue: string[]; }] | [event: "progress", data: { id: string; kind: "stderr"; message: string; }] | [event: "progress", data: { ...; }] | ... 4 more ... | [event: ...]'.
    Type '["progress", OperationProgress]' is not assignable to type '[event: "progress", data: { id: string; kind: "forgot"; }]'.
      Type at position 1 in source is not compatible with type at position 1 in target.
        Type 'OperationProgress' is not assignable to type '{ id: string; kind: "forgot"; }'.
          Type '{ id: string; kind: "queue"; queue: string[]; }' is not assignable to type '{ id: string; kind: "forgot"; }'.
            Types of property 'kind' are incompatible.
              Type '"queue"' is not assignable to type '"forgot"'.ts(2345)

This diff changes the signature to make it easier for tsc to understand.

Reviewed By: evangrayk

Differential Revision: D54364589

fbshipit-source-id: 841dd82449d1b21a2287b3a36b755b081094280d

addons/shared/TypedEventEmitter.ts

@@ -58,14 +58,11 @@ export class TypedEventEmitter<EventName extends string, EventType> {
     return this;
   }
 
-  emit(
-    ...args:
-      | [event: 'error', data: Error]
-      | (EventType extends undefined
-          ? [event: EventName] | [event: EventName, data: EventType]
-          : [event: EventName, data: EventType])
-  ): boolean {
-    const [name, data] = args;
+  emit(event: EventName): EventType extends undefined ? boolean : never;
+  emit(event: EventName, data: EventType): boolean;
+  emit(event: 'error', data: Error): boolean;
+
+  emit(name: EventName | 'error', data?: EventType | Error): boolean {
     const event = new EventWithPayload(name, data);
     if (!this.target.dispatchEvent(event)) {
       return false;