Add support for rethrow and start treating throw <expr> as an expression.

sdk/lib/_internal/compiler/implementation/dart_backend/utils.dart

Index: sdk/lib/_internal/compiler/implementation/dart_backend/utils.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/utils.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/utils.dart
index c30a2cfab6b5ea704096861fdaff2e97153ff9d5..3dbc248930b7d8966269b28254c34df18eb988dd 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/utils.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/utils.dart
@@ -163,6 +163,9 @@ class CloningVisitor implements Visitor<Node> {
   visitParenthesizedExpression(ParenthesizedExpression node) =>
       new ParenthesizedExpression(visit(node.expression), node.beginToken);
 
+  visitRethrow(Rethrow node) => new Rethrow(
+      node.throwToken, node.endToken);
+
   visitReturn(Return node) => new Return(
       node.beginToken, node.endToken, visit(node.expression));