src/vs/workbench/contrib/extensions/browser/extensionsWorkbenchService.ts/Extension/getChangelog
class Extension:	getChangelog(token: CancellationToken): Promise<string> {

		if (this.local && this.local.changelogUrl && !this.outdated) {
			return this.fileService.readFile(this.local.changelogUrl).then(content => content.value.toString());
		}

		if (this.gallery && this.gallery.assets.changelog) {
			return this.galleryService.getChangelog(this.gallery, token);
		}

		if (this.type === ExtensionType.System) {
			return Promise.resolve('Please check the [VS Code Release Notes](command:update.showCurrentReleaseNotes) for changes to the built-in extensions.');
		}

		return Promise.reject(new Error('not available'));
	}