Add parser support for generators.

src/prettyprinter.cc

Index: src/prettyprinter.cc
diff --git a/src/prettyprinter.cc b/src/prettyprinter.cc
index df6183a3662b8e4416034cb1cb0075dc02da4d68..50a71ced3d2998fd9c4fad68ceb606e206f2beb8 100644
--- a/src/prettyprinter.cc
+++ b/src/prettyprinter.cc
@@ -353,6 +353,12 @@ void PrettyPrinter::VisitAssignment(Assignment* node) {
 }
 
 
+void PrettyPrinter::VisitYield(Yield* node) {
+  Print("yield ");
+  Visit(node->expression());
+}
+
+
 void PrettyPrinter::VisitThrow(Throw* node) {
   Print("throw ");
   Visit(node->exception());
@@ -1059,6 +1065,11 @@ void AstPrinter::VisitAssignment(Assignment* node) {
 }
 
 
+void AstPrinter::VisitYield(Yield* node) {
+  PrintIndentedVisit("YIELD", node->expression());
+}
+
+
 void AstPrinter::VisitThrow(Throw* node) {
   PrintIndentedVisit("THROW", node->exception());
 }