RegExp_ExecuteMatch should return List<int> in strong mode

[stereotype441]

I'm working on a CL that adds strong mode type checks to invocations. The CL is currently failing tests like language_2/reg_ex2_test with exceptions like this:

Unhandled exception:
type 'List' is not a subtype of type 'List<int>' where
  List is from dart:core
  List is from dart:core
  int is from dart:core

#0      _RegExp.firstMatch (dart:core)
#1      RegEx2Test.testMain (file:///usr/local/google/home/paulberry/dart1/sdk/tests/language_2/reg_ex2_test.dart:10:32)
#2      main (file:///usr/local/google/home/paulberry/dart1/sdk/tests/language_2/reg_ex2_test.dart:25:14)
#3      _startIsolate.<anonymous closure> (dart:isolate:1252:19)
#4      _RawReceivePortImpl._handleMessage (dart:isolate:1142:12)

For reference, the definition of firstMatch is:

  Match firstMatch(String str) {
    if (str is! String) throw new ArgumentError(str);
    List match = _ExecuteMatch(str, 0);
    if (match == null) {
      return null;
    }
    return new _RegExpMatch(this, str, match);
  }

and the definition of _ExecuteMatch is:

  List _ExecuteMatch(String str, int start_index) native "RegExp_ExecuteMatch";

The problem is that _RegExpMatch requires its third argument to be List<int>, but RegExp_ExecuteMatch apparently returns List.

An alternative fix would be to change the type of _RegExpMatch._match to List, however this would be less performant because it would incur an implicit downcast in _RegExpMatch._start and _RegExpMatch._end.