Improve exception handling:

dart/frog/leg/ssa/nodes.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.
 
 interface HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
@@ skipping 2066 matching lines @@
   bool dataEquals(HInstruction other) => true;
 }
 
 class HReturn extends HControlFlow {
   HReturn(value) : super(<HInstruction>[value]);
   toString() => 'return';
   accept(HVisitor visitor) => visitor.visitReturn(this);
 }
 
 class HThrow extends HControlFlow {
-  HThrow(value) : super(<HInstruction>[value]);
+  final bool isRethrow;
+  HThrow(value, [this.isRethrow = false]) : super(<HInstruction>[value]);
   toString() => 'throw';
   accept(HVisitor visitor) => visitor.visitThrow(this);
 }
 
 class HStatic extends HInstruction {
   Element element;
   HStatic(this.element) : super(<HInstruction>[]) {
     tryGenerateAtUseSite();
   }
   void prepareGvn() {
@@ skipping 155 matching lines @@
 
   HInstruction get expression() => inputs[0];
 
   HType computeType() => HType.BOOLEAN;
   bool hasExpectedType() => true;
 
   accept(HVisitor visitor) => visitor.visitIs(this);
 
   toString() => "$expression is $typeExpression";
 }