Add void operator to fast compiler.

src/x64/fast-codegen-x64.cc

Index: src/x64/fast-codegen-x64.cc
===================================================================
--- src/x64/fast-codegen-x64.cc	(revision 3184)
+++ src/x64/fast-codegen-x64.cc	(working copy)
@@ -741,6 +741,31 @@
 }
 
 
+void FastCodeGenerator::VisitUnaryOperation(UnaryOperation* expr) {
+  Comment cmnt(masm_, "[ UnaryOperation");
+
+  switch (expr->op()) {
+    case Token::VOID:
+      Visit(expr->expression());
+      ASSERT_EQ(Expression::kEffect, expr->expression()->context());
+      switch (expr->context()) {
+        case Expression::kUninitialized:
+          UNREACHABLE();
+          break;
+        case Expression::kValue:
+          __ PushRoot(Heap::kUndefinedValueRootIndex);
+          break;
+        case Expression::kEffect:
+          break;
+      }
+      break;
+
+    default:
+      UNREACHABLE();
+  }
+}
+
+
 void FastCodeGenerator::VisitBinaryOperation(BinaryOperation* expr) {
   switch (expr->op()) {
     case Token::COMMA: