code: @GwtIncompatible("System.getEnv")
    @Nullable
    public static TrustedResourceUrl fromEnvironmentVariable(
        @CompileTimeConstant final String variableName) {
      String var = System.getenv(variableName);
      if (var == null) {
        return null;
      }
      return create(var);
    }
docs: Creates a {@link TrustedResourceUrl} from the value of an environment variable. In a server
  setting, environment variables are part of the application's deployment configuration and are
  hence considered application-controlled. If the variable is not defined returns null.
  
  
  <p>No runtime validation or sanitization is performed on the value of the environment variable;
  being under application control, it is simply assumed to comply with the TrustedResourceUrl
  contract.
  
  @throws SecurityException  if a security manager exists and its checkPermission method doesn't
  allow access to the environment variable name