code: public java.lang.String getExternalValuesOrThrow(
        java.lang.String key) {
      if (key == null) { throw new java.lang.NullPointerException(); }
      java.util.Map<java.lang.String, java.lang.String> map =
          internalGetExternalValues().getMap();
      if (!map.containsKey(key)) {
        throw new java.lang.IllegalArgumentException();
      }
      return map.get(key);
    }
docs: <pre>
  Value names referenced by but external to this context.
  </pre>
  
  <code>map&lt;string, string&gt; external_values = 2;</code>