Ensure integrity of ASCII strings.

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index 9006abde1b88114f9ab57f8554993d5b042f2bf4..89b25bdbba10ffca3f6cce8da465ebbe072d854a 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -458,10 +458,17 @@ void String::StringVerify() {
     ConsString::cast(this)->ConsStringVerify();
   } else if (IsSlicedString()) {
     SlicedString::cast(this)->SlicedStringVerify();
+  } else if (IsSeqAsciiString()) {
+    SeqAsciiString::cast(this)->SeqAsciiStringVerify();
   }
 }
 
 
+void SeqAsciiString::SeqAsciiStringVerify() {
+  CHECK(String::IsAscii(GetChars(), length()));
+}
+
+
 void ConsString::ConsStringVerify() {
   CHECK(this->first()->IsString());
   CHECK(this->second() == GetHeap()->empty_string() ||