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

sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart

Index: sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
diff --git a/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart b/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
index 62b67441f631eae14c703d8593a9081efaa35256..9f2cfbf325a8d443ea4b675f766bd8f7f642544b 100644
--- a/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
+++ b/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
@@ -305,6 +305,10 @@ class PrettyPrinter implements Visitor {
     visitNodeWithChildren(node, "ParenthesizedExpression");
   }
 
+  visitRethrow(Rethrow node) {
+    visitNodeWithChildren(node, "Rethrow");
+  }
+
   visitReturn(Return node) {
     openNode(node, "Return");
     visitChildNode(node.expression, "expression");