output: allow to dispose output channels

fixes #20703

src/vs/workbench/api/node/extHost.protocol.ts

@@ -183,6 +183,7 @@ export abstract class MainThreadMessageServiceShape {
 export abstract class MainThreadOutputServiceShape {
 	$append(channelId: string, label: string, value: string): TPromise<void> { throw ni(); }
 	$clear(channelId: string, label: string): TPromise<void> { throw ni(); }
+	$dispose(channelId: string, label: string): TPromise<void> { throw ni(); }
 	$reveal(channelId: string, label: string, preserveFocus: boolean): TPromise<void> { throw ni(); }
 	$close(channelId: string): TPromise<void> { throw ni(); }
 }

src/vs/workbench/api/node/extHostOutputService.ts

@@ -29,7 +29,7 @@ export class ExtHostOutputChannel implements vscode.OutputChannel {
 
 	dispose(): void {
 		if (!this._disposed) {
-			this._proxy.$clear(this._id, this._name).then(() => {
+			this._proxy.$dispose(this._id, this._name).then(() => {
 				this._disposed = true;
 			});
 		}

src/vs/workbench/api/node/mainThreadOutputService.ts

@@ -58,4 +58,9 @@ export class MainThreadOutputService extends MainThreadOutputServiceShape {
 
 		return undefined;
 	}
+
+	public $dispose(channelId: string, label: string): TPromise<void> {
+		this._getChannel(channelId, label).dispose();
+		return undefined;
+	}
 }