int lastIndexOf(Pattern pattern, [int start])

Returns the position of the last match pattern in this string, searching backward starting at start, inclusive:

var string = 'Dartisans';
string.lastIndexOf('a');                    // 6
string.lastIndexOf(new RegExp(r'a(r|n)'));  // 6

Returns -1 if other could not be found.

string.lastIndexOf(new RegExp(r'DART'));    // -1

start must not be negative or greater than length.

Source 

int lastIndexOf(Pattern pattern, [int start]);