Use direct copy and templates to speed up flattening of strings.

src/utils.h

Index: src/utils.h
===================================================================
--- src/utils.h	(revision 535)
+++ src/utils.h	(working copy)
@@ -443,6 +443,19 @@
   DISALLOW_IMPLICIT_CONSTRUCTORS(StringBuilder);
 };
 
+
+// Widening copy from ASCII to 16 bit chars.  This is templatized to allow
+// it to be used inside if statements in templatized callers, but it is only
+// usable for sinkchar == uc16.
+template <typename sinkchar>
+static inline void Cpy168(sinkchar* dest, const unsigned char* src, int chars) {
+  ASSERT(sizeof(sinkchar) == 2);
+  while (chars--) {
+    char c = *src++;
+    *dest++ = c;
+  }
+}
+
 } }  // namespace v8::internal
 
 #endif  // V8_UTILS_H_