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

lib/compiler/implementation/tree/nodes.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 interface Visitor<R> {
   R visitBlock(Block node);
   R visitBreakStatement(BreakStatement node);
   R visitCascade(Cascade node);
   R visitCascadeReceiver(CascadeReceiver node);
   R visitCaseMatch(CaseMatch node);
@@ skipping 1684 matching lines @@
   final Token catchKeyword;
 
   CatchBlock(this.type, this.formals, this.block,
              this.onKeyword, this.catchKeyword);
 
   CatchBlock asCatchBlock() => this;
 
   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;
     return head.definitions.nodes.head;
   }
 
   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);
   }
 
   Token getBeginToken() => onKeyword != null ? onKeyword : catchKeyword;
 
   Token getEndToken() => block.getEndToken();
 }
 
 class Initializers {
   static bool isSuperConstructorCall(Send node) {
@@ skipping 38 matching lines @@
  * argument).
  *
  * TODO(ahe): This method is controversial, the team needs to discuss
  * if top-level methods are acceptable and what naming conventions to
  * use.
  */
 initializerDo(Node node, f(Node node)) {
   SendSet send = node.asSendSet();
   if (send !== null) return f(send.arguments.head);
 }