src/vs/workbench/services/decorations/browser/decorationsService.ts/DecorationsService/_ensureEntry
class DecorationsService:	private _ensureEntry(uri: URI): DecorationEntry {
		let map = this._data.get(uri);
		if (!map) {
			// nothing known about this uri
			map = new Map();
			this._data.set(uri, map);
		}
		return map;
	}