Implement [Generator].prototype.return.

src/runtime/runtime-generator.cc

Index: src/runtime/runtime-generator.cc
diff --git a/src/runtime/runtime-generator.cc b/src/runtime/runtime-generator.cc
index 54894a013a624d287fe4efe03e04bbc678410527..2999062d14c3ba2d5d8b8d4c38149f3e9e734db1 100644
--- a/src/runtime/runtime-generator.cc
+++ b/src/runtime/runtime-generator.cc
@@ -87,9 +87,9 @@ RUNTIME_FUNCTION(Runtime_SuspendJSGeneratorObject) {
 // called if the suspended activation had operands on the stack, stack handlers
 // needing rewinding, or if the resume should throw an exception.  The fast path
 // is handled directly in FullCodeGenerator::EmitGeneratorResume(), which is
-// inlined into GeneratorNext and GeneratorThrow.  EmitGeneratorResumeResume is
-// called in any case, as it needs to reconstruct the stack frame and make space
-// for arguments and operands.
+// inlined into GeneratorNext, GeneratorReturn, and GeneratorThrow.
+// EmitGeneratorResume is called in any case, as it needs to reconstruct the
+// stack frame and make space for arguments and operands.
 RUNTIME_FUNCTION(Runtime_ResumeJSGeneratorObject) {
   SealHandleScope shs(isolate);
   DCHECK(args.length() == 3);
@@ -125,7 +125,10 @@ RUNTIME_FUNCTION(Runtime_ResumeJSGeneratorObject) {
   JSGeneratorObject::ResumeMode resume_mode =
       static_cast<JSGeneratorObject::ResumeMode>(resume_mode_int);
   switch (resume_mode) {
+    // Note: this looks like NEXT and RETURN are the same but RETURN receives
+    // special treatment in the generator code (to which we return here).
     case JSGeneratorObject::NEXT:
+    case JSGeneratorObject::RETURN:
       return value;
     case JSGeneratorObject::THROW:
       return isolate->Throw(value);
@@ -212,6 +215,12 @@ RUNTIME_FUNCTION(Runtime_GeneratorNext) {
 }
 
 
+RUNTIME_FUNCTION(Runtime_GeneratorReturn) {
+  UNREACHABLE();  // Optimization disabled in SetUpGenerators().
+  return NULL;
+}
+
+
 RUNTIME_FUNCTION(Runtime_GeneratorThrow) {
   UNREACHABLE();  // Optimization disabled in SetUpGenerators().
   return NULL;