src/vs/workbench/services/configurationResolver/common/variableResolver.ts/AbstractVariableResolverService/contributeVariable
class AbstractVariableResolverService:	public contributeVariable(variable: string, resolution: () => Promise<string | undefined>): void {
		if (this._contributedVariables.has(variable)) {
			throw new Error('Variable ' + variable + ' is contributed twice.');
		} else {
			this._contributedVariables.set(variable, resolution);
		}
	}