Programs in UTF-8 with no declared coding are accepted

If a program's source uses UTF-8, but the file's encoding is not declared using a `coding` declaration, Pyflakes accepts the program as valid, even though Python itself refuses it.

$ file coding.py
coding.py: UTF-8 Unicode text
$ cat coding.py
print "¡this file contains ŭnicode!"
$ pyflakes --version
1.2.3
$ pyflakes coding.py
$ python coding.py
  File "coding.py", line 1
SyntaxError: Non-ASCII character '\xc2' in file coding.py on line 1, but no encoding declared; see http://python.org/dev/peps/pep-0263/ for details
$

Expected outcome: Pyflakes should report a syntax error for such files.