fixes #221, initial sync*, async, async* implementation

lib/src/js/nodes.dart

Index: lib/src/js/nodes.dart
diff --git a/lib/src/js/nodes.dart b/lib/src/js/nodes.dart
index 7cff60c69151aeac024e38d60521fac762d789c4..62fe2e6794fe40bf2bff0b563b0e949065ff394f 100644
--- a/lib/src/js/nodes.dart
+++ b/lib/src/js/nodes.dart
@@ -1282,7 +1282,7 @@ class TaggedTemplate extends Expression {
 
 // TODO(jmesserly): parser does not support this yet.
 class Yield extends Expression {
-  final Expression value;
+  final Expression value; // Can be null.
 
   /**
    * Whether this yield expression is a `yield*` that iterates each item in
@@ -1295,7 +1295,7 @@ class Yield extends Expression {
   accept(NodeVisitor visitor) => visitor.visitYield(this);
 
   void visitChildren(NodeVisitor visitor) {
-    value.accept(visitor);
+    if (value != null) value.accept(visitor);
   }
 
   Yield _clone() => new Yield(value);