Fix issue 5517 by setting the successors the right way in a try/catch.

lib/compiler/implementation/ssa/validate.dart

Index: lib/compiler/implementation/ssa/validate.dart
===================================================================
--- lib/compiler/implementation/ssa/validate.dart	(revision 12963)
+++ lib/compiler/implementation/ssa/validate.dart	(working copy)
@@ -37,7 +37,7 @@
       markInvalid("Conditional node without two successors");
     }
     if (block.last is HGoto && block.successors.length != 1) {
-      markInvalid("Goto node without one successor");
+      markInvalid("Goto node with not exactly one successor");
     }
     if (block.last is HJump && block.successors.length != 1) {
       markInvalid("Break or continue node without one successor");