Check for null args in the mojo:dart_wget example.
Check for null args in the mojo:dart_wget example.

Without this check, this less useful exception is raised:
  Uncaught Error: The null object does not have a getter 'length'.
With the new check, the expected exception message is printed:
  Uncaught Error: Expected URL argument

BUG=NONE
R=zra@google.com
TEST="mojo_shell mojo:dart_wget" prints the expected argument error.

Review URL: https://codereview.chromium.org/873433003

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