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

lib/compiler/implementation/ssa/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.
 
 abstract class HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
@@ skipping 563 matching lines @@
 
   void remove(HInstruction instruction) {
     assert(isOpen() || isClosed());
     assert(instruction is !HPhi);
     super.remove(instruction);
     assert(instruction.block == this);
     instruction.notifyRemovedFromBlock();
   }
 
   void addSuccessor(HBasicBlock block) {
-    // Forward branches are only allowed to new blocks.
-    assert(isClosed() && (block.isNew() || block.id < id));
     if (successors.isEmpty()) {
       successors = [block];
     } else {
       successors.add(block);
     }
     block.predecessors.add(this);
   }
 
   void postProcessLoopHeader() {
     assert(isLoopHeader());
@@ skipping 2323 matching lines @@
   HBasicBlock get start => expression.start;
   HBasicBlock get end {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty());
     return statements.last().end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }