Change dart2js to follow the new spec on the equality operator.

Change dart2js to follow the new spec on the equality operator.

'null' is only equals to 'null' and nothing else. The user-defined
operator== is never called with null.

This also means that a 'a == null' and 'null == a' can always be
treated as a builtin when compiling to javascript.

Update the test expectations for co19. I will file co19 bugs about
updating the tests.

R=floitsch@google.com
BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=9348

[floitsch, 2012-07-03 13:35:29]

LGTM.

https://chromiumcodereview.appspot.com/10697068/diff/1/tests/language/operato...
File tests/language/operator5_test.dart (right):

https://chromiumcodereview.appspot.com/10697068/diff/1/tests/language/operato...
tests/language/operator5_test.dart:57: equalsNull(a);
hehe.