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

/**
 * 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].
 */
int lastIndexOf(Pattern pattern, [int start]);