Fix parent pointer issue in async rewriter.

lib/ast.dart

Index: lib/ast.dart
diff --git a/lib/ast.dart b/lib/ast.dart
index ab0a3b5356f9f1a293343ade2ed21479d8d2273a..b8f08976ecb666e2f0f822969922e2133ff45f14 100644
--- a/lib/ast.dart
+++ b/lib/ast.dart
@@ -2455,6 +2455,11 @@ class Block extends Statement {
   transformChildren(Transformer v) {
     transformList(statements, v, this);
   }
+
+  void addStatement(Statement node) {
+    statements.add(node);
+    node.parent = this;
+  }
 }
 
 class EmptyStatement extends Statement {