lastIndexOf method
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 pattern could not be found in this string.
string.lastIndexOf(new RegExp(r'DART'));    // -1
The start must be non-negative and not greater than length.
Implementation
int lastIndexOf(Pattern pattern, [int start]);