Allow catch part to be optional as in try {} on T [catch (x[,y])] {}

lib/compiler/implementation/tree/nodes.dart

Index: lib/compiler/implementation/tree/nodes.dart
diff --git a/lib/compiler/implementation/tree/nodes.dart b/lib/compiler/implementation/tree/nodes.dart
index 9cc59532946740808c755eac82d62c4c52e2969c..d8a7ab1ba59b6bb05e0c4a30e53c2fe645c5d66e 100644
--- a/lib/compiler/implementation/tree/nodes.dart
+++ b/lib/compiler/implementation/tree/nodes.dart
@@ -1702,13 +1702,13 @@ class CatchBlock extends Node {
   accept(Visitor visitor) => visitor.visitCatchBlock(this);
 
   Node get exception {
-    if (formals.nodes.isEmpty()) return null;
+    if (formals === null || formals.nodes.isEmpty()) return null;
     VariableDefinitions declarations = formals.nodes.head;
     return declarations.definitions.nodes.head;
   }
 
   Node get trace {
-    if (formals.nodes.isEmpty()) return null;
+    if (formals === null || formals.nodes.isEmpty()) return null;
     Link<Node> declarations = formals.nodes.tail;
     if (declarations.isEmpty()) return null;
     VariableDefinitions head = declarations.head;
@@ -1716,8 +1716,8 @@ class CatchBlock extends Node {
   }
 
   visitChildren(Visitor visitor) {
-    if (type != null) type.accept(visitor);
-    formals.accept(visitor);
+    if (type !== null) type.accept(visitor);
+    if (formals !== null) formals.accept(visitor);
     block.accept(visitor);
   }