Revive throw as an expression but not for rethrow.

sdk/lib/_internal/compiler/implementation/ssa/validate.dart

 // Copyright (c) 2011, 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.
 
 part of ssa;
 
 class HValidator extends HInstructionVisitor {
   bool isValid = true;
   HGraph graph;
 
@@ skipping 33 matching lines @@
     if (block.last is HJump && block.successors.length != 1) {
       markInvalid("Break or continue node without one successor");
     }
     if (block.last is HReturn &&
         (block.successors.length != 1 || !block.successors[0].isExitBlock())) {
       markInvalid("Return node with > 1 succesor or not going to exit-block");
     }
     if (block.last is HExit && !block.successors.isEmpty) {
       markInvalid("Exit block with successor");
     }
-    if (block.last is HThrow && !block.successors.isEmpty) {
-      markInvalid("Throw block with successor");
+    if (block.last is HThrow && block.successors.length > 1) {
+      markInvalid("Throw block with more than one successor");
     }
 
     if (block.successors.isEmpty &&
         block.last is !HThrow &&
         !block.isExitBlock()) {
       markInvalid("Non-exit or throw block without successor");
     }
 
     // Check that successors ids are always higher than the current one.
     // TODO(floitsch): this is, of course, not true for back-branches.
@@ skipping 106 matching lines @@
       markInvalid("Instruction in wrong block");
     }
     if (!hasCorrectInputs()) {
       markInvalid("Incorrect inputs");
     }
     if (!hasCorrectUses()) {
       markInvalid("Incorrect uses");
     }
   }
 }