String replaceFirst(Pattern from, String to, [int startIndex = 0])

Returns a new string in which the first occurence of from in this string is replaced with to, starting from startIndex:

'0.0001'.replaceFirst(new RegExp(r'0'), ''); // '.0001'
'0.0001'.replaceFirst(new RegExp(r'0'), '7', 1); // '0.7001'

Source 

String replaceFirst(Pattern from, String to, [int startIndex = 0]);