Fix mirror-script and debug tests when using external natives.

src/natives-external.cc

Index: src/natives-external.cc
diff --git a/src/natives-external.cc b/src/natives-external.cc
index cfa00bc1345a1cb746d8946ceeb272cf7d444eeb..75e521854d3743fe9f096d991b0b587fc167951c 100644
--- a/src/natives-external.cc
+++ b/src/natives-external.cc
@@ -82,6 +82,10 @@ class NativesStore {
     builder.AddString("native ");
     builder.AddSubstring(reinterpret_cast<const char*>(id), id_length);
     builder.AddString(".js");
+    builder.Finalize();
+    // SimpleStringBuilder wants zero-byte; the caller does not.
+    DCHECK(name[name.length() - 1] == '\0');
+    name.Truncate(name.length() - 1);
     return Vector<const char>::cast(name);
   }