add function to test whether string contents are definitely one byte

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index ca1c65065085d084378cde64d4a7d76550af67da..24c60ece90226acc377365514820761cf81242d4 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -1205,6 +1205,11 @@ bool String::MakeExternal(v8::String::ExternalAsciiStringResource* resource) {
   if (FLAG_enable_slow_asserts) {
     // Assert that the resource and the string are equivalent.
     ASSERT(static_cast<size_t>(this->length()) == resource->length());
+    if (this->IsTwoByteRepresentation()) {
+      ScopedVector<uint16_t> smart_chars(this->length());
+      String::WriteToFlat(this, smart_chars.start(), 0, this->length());
+      ASSERT(String::IsOneByte(smart_chars.start(), this->length()));
+    }

[Yang, 2013/06/06 12:57:36]

Is this really necessary? We compare the new resource with the old string
content right below. If the old (possibly two-byte) string has non-one-byte
chars, that would fail already.

[dcarney, 2013/06/06 13:11:03]

the comparison below only compares lower bytes of the string

     ScopedVector<char> smart_chars(this->length());
     String::WriteToFlat(this, smart_chars.start(), 0, this->length());
     ASSERT(memcmp(smart_chars.start(),