Fix try catch crasher bug due to null else case.

lib/src/js/nodes.dart

Index: lib/src/js/nodes.dart
diff --git a/lib/src/js/nodes.dart b/lib/src/js/nodes.dart
index 9e2553fb9215fcaf3604764828d77e0458b70859..47c1155a4b7123c10c0ed6032aafa70752e8f1e3 100644
--- a/lib/src/js/nodes.dart
+++ b/lib/src/js/nodes.dart
@@ -275,7 +275,8 @@ class If extends Statement {
   final Node then;
   final Node otherwise;
 
-  If(this.condition, this.then, this.otherwise);
+  If(this.condition, this.then, Node otherwise) :

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

We could alternately make this constructor assert if otherwise is null but I
think that will just make upstream code less convenient to write.

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

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.

+      otherwise = otherwise != null ? otherwise : new EmptyStatement();
   If.noElse(this.condition, this.then) : this.otherwise = new EmptyStatement();
 
   bool get hasElse => otherwise is !EmptyStatement;