Fix try catch crasher bug due to null else case.

Fix try catch crasher bug due to null else case.

BUG=
R=jmesserly@google.com

Committed: https://github.com/dart-lang/dev_compiler/commit/d7ac315b11fb471f14b3e6fc36d1256f6f71d749

[Jacob, 2015-03-27 17:04:30]

jacobr@google.com changed reviewers:
+ jmesserly@google.com

[Jacob, 2015-03-27 17:04:31]

https://codereview.chromium.org/1036333003/diff/1/lib/src/js/nodes.dart
File lib/src/js/nodes.dart (right):

https://codereview.chromium.org/1036333003/diff/1/lib/src/js/nodes.dart#newco...
lib/src/js/nodes.dart:278: If(this.condition, this.then, Node otherwise) :
We could alternately make this constructor assert if otherwise is null but I
think that will just make upstream code less convenient to write.

https://codereview.chromium.org/1036333003/diff/1/test/codegen/try_catch.dart
File test/codegen/try_catch.dart (right):

https://codereview.chromium.org/1036333003/diff/1/test/codegen/try_catch.dart...
test/codegen/try_catch.dart:34: throw "on only";
code that triggers a NPE without the fix.

[Jennifer Messerly, 2015-03-27 17:15:54]

with this fix, I think you can also remove _visitOrEmpty in js_codegen.

https://codereview.chromium.org/1036333003/diff/1/lib/src/js/nodes.dart
File lib/src/js/nodes.dart (right):

https://codereview.chromium.org/1036333003/diff/1/lib/src/js/nodes.dart#newco...
lib/src/js/nodes.dart:278: If(this.condition, this.then, Node otherwise) :
On 2015/03/27 17:04:30, Jacob wrote:
> We could alternately make this constructor assert if otherwise is null but I
> think that will just make upstream code less convenient to write.

Agreed. Another option would be to just have null `otherwise` and change
Printer, as most other AST nodes here use `null` to mean missing. But this looks
good too.

[Jennifer Messerly, 2015-03-27 17:16:04]

also lgtm

[Jacob, 2015-03-27 17:38:43]

On 2015/03/27 17:16:04, John Messerly wrote:
> also lgtm

changed to null meaning missing.

[Jennifer Messerly, 2015-03-27 17:53:24]

lgtm

[Jacob, 2015-03-27 17:53:58]

Committed patchset #3 (id:40001) manually as
d7ac315b11fb471f14b3e6fc36d1256f6f71d749 (presubmit successful).