Non-matched group should return null

[iposva-google]

This test fails on the VM

main() {
 RegExp re = new RegExp('(a)?b');
 Match m = re.firstMatch('b');
 print('The full match is ${m[0]}');
 print('The first group matched ${m[1]}');
 Expect.isNull(m[1]);
}

[lexprfuncall]

http://codereview.chromium.org/9718015/

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Added Fixed label.