Fix mac dartium build

runtime/bin/utils_macos.cc

Index: runtime/bin/utils_macos.cc
diff --git a/runtime/bin/utils_macos.cc b/runtime/bin/utils_macos.cc
index 2421d2a6a8e09325c762b37713fe868775050fd5..97717c756e6e26c66ae52c2e7e83bbf5a96b2993 100644
--- a/runtime/bin/utils_macos.cc
+++ b/runtime/bin/utils_macos.cc
@@ -78,7 +78,26 @@ char* StringUtils::Utf8ToConsoleString(
 
 
 char* StringUtils::StrNDup(const char* s, intptr_t n) {
+  // strndup has only been added to Mac OS X in 10.7. We are supplying
+  // our own copy here if needed.
+#if !defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || \
+    __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ <= 1060
+  intptr_t len = strlen(s);
+  if ((n < 0) || (len < 0)) {
+    return NULL;
+  }
+  if (n < len) {
+    len = n;
+  }
+  char* result = reinterpret_cast<char*>(malloc(len + 1));
+  if (result == NULL) {
+    return NULL;
+  }
+  result[len] = '\0';
+  return reinterpret_cast<char*>(memmove(result, s, len));
+#else  // !defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || ...
   return strndup(s, n);
+#endif  // !defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__) || ...
 }