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

lib/compiler/implementation/ssa/builder.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.
 
 class Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 3654 matching lines @@
             }
             condition = new HIs(type, unwrappedException, nullOk: true);
             push(condition);
           }
         }
       }
 
       void visitThen() {
         CatchBlock catchBlock = link.head;
         link = link.tail;
-        localsHandler.updateLocal(elements[catchBlock.exception],
-                                  unwrappedException);
+        if (catchBlock.exception !== null) {
+          localsHandler.updateLocal(elements[catchBlock.exception],
+                                    unwrappedException);
+        }
         Node trace = catchBlock.trace;
         if (trace != null) {
           pushInvokeHelper1(interceptors.getTraceFromException(), exception);
           HInstruction traceInstruction = pop();
           localsHandler.updateLocal(elements[trace], traceInstruction);
         }
         visit(catchBlock);
       }
 
       void visitElse() {
@@ skipping 451 matching lines @@
           new HSubGraphBlockInformation(elseBranch.graph));
 
     HBasicBlock conditionStartBlock = conditionBranch.block;
     conditionStartBlock.setBlockFlow(info, joinBlock);
     SubGraph conditionGraph = conditionBranch.graph;
     HIf branch = conditionGraph.end.last;
     assert(branch is HIf);
     branch.blockInformation = conditionStartBlock.blockFlow;
   }
 }