Flatten the string in StringToDouble function.

src/conversions.cc

Index: src/conversions.cc
diff --git a/src/conversions.cc b/src/conversions.cc
index 8b776235e7b255a8638b954856bfcab159665a2f..663f4e8dec254c4126142dc2b5e628e5a8aad97c 100644
--- a/src/conversions.cc
+++ b/src/conversions.cc
@@ -483,19 +483,21 @@ char* DoubleToRadixCString(double value, int radix) {
 }
 
 
-double StringToDouble(UnicodeCache* unicode_cache,
-                      String* string,
-                      int flags,
-                      double empty_string_val) {
-  DisallowHeapAllocation no_gc;
-  String::FlatContent flat = string->GetFlatContent();
-  // ECMA-262 section 15.1.2.3, empty string is NaN
-  if (flat.IsOneByte()) {
-    return StringToDouble(
-        unicode_cache, flat.ToOneByteVector(), flags, empty_string_val);
-  } else {
-    return StringToDouble(
-        unicode_cache, flat.ToUC16Vector(), flags, empty_string_val);
+double StringToDouble(UnicodeCache* unicode_cache, Handle<String> string,
+                      int flags, double empty_string_val) {
+  Handle<String> flattened = String::Flatten(string);
+  {
+    DisallowHeapAllocation no_gc;
+    String::FlatContent flat = flattened->GetFlatContent();
+    DCHECK(flat.IsFlat());
+    // ECMA-262 section 15.1.2.3, empty string is NaN
+    if (flat.IsOneByte()) {
+      return StringToDouble(unicode_cache, flat.ToOneByteVector(), flags,
+                            empty_string_val);
+    } else {
+      return StringToDouble(unicode_cache, flat.ToUC16Vector(), flags,
+                            empty_string_val);
+    }
   }
 }