dart2js cps: Support async/await by rewriting the JavaScript AST.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
index 1a317c4d454f0aefa2259152ab4f52fd8c353c56..d37132e42a841456648b9f622bf61c9db403ee29 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
@@ -350,6 +350,13 @@ class SExpressionStringifier extends Indentation implements Visitor<String> {
     String value = access(node.value);
     return '(SetIndex $object $index $value)';
   }
+
+  @override
+  String visitAwait(Await node) {
+    String value = access(node.input);
+    String continuation = access(node.continuation);
+    return '(Await $value $continuation)';
+  }
 }
 
 class ConstantStringifier extends ConstantValueVisitor<String, Null> {