dart2js cps: Support sync* and yield.

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 2ae63c2bd76a88763c59b1d7d3631fa953354d37..476f8b623e7f24761b813c928324a297af1ae3d4 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
@@ -367,6 +367,12 @@ class SExpressionStringifier extends Indentation implements Visitor<String> {
   }
 
   @override
+  String visitYield(Yield node) {
+    String value = access(node.input);
+    String continuation = access(node.continuation);
+    return '(Yield $value $continuation)';
+  }
+
   String visitRefinement(Refinement node) {
     String value = access(node.value);
     return '(Refinement $value ${node.type})';