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

lib/compiler/implementation/tree/unparser.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.
 
 // Returns null if no need to rename a node.
 typedef String Renamer(Node node);
 
 String unparse(Node node) {
   Unparser unparser = new Unparser();
   unparser.unparse(node);
@@ skipping 455 matching lines @@
     if (node.prefixIdentifier !== null) {
       visit(node.prefixIdentifier);
       sb.add(':');
       visit(node.prefix);
     }
     sb.add(')');
     add(node.endToken.value);
   }
 
   visitTryStatement(TryStatement node) {
-    add(node.tryKeyword.value);
+    addToken(node.tryKeyword);
     visit(node.tryBlock);
     visit(node.catchBlocks);
     if (node.finallyKeyword !== null) {
       addToken(node.finallyKeyword);
       visit(node.finallyBlock);
     }
   }
 
   visitCaseMatch(CaseMatch node) {
     add(node.caseKeyword.value);
     sb.add(" ");
     visit(node.expression);
     add(node.colonToken.value);
   }
 
   visitCatchBlock(CatchBlock node) {
     addToken(node.onKeyword);
     if (node.type !== null) {
       visit(node.type);
       sb.add(' ');
     }
-    add(node.catchKeyword.value);
+    addToken(node.catchKeyword);
     visit(node.formals);
     visit(node.block);
   }
 
   visitTypedef(Typedef node) {
     addToken(node.typedefKeyword);
     if (node.returnType !== null) {
       visit(node.returnType);
       sb.add(' ');
     }
     visit(node.name);
     if (node.typeParameters !== null) {
       visit(node.typeParameters);
     }
     visit(node.formals);
     add(node.endToken.value);
   }
 }