Beijing/northern colloquialism with an erhua ending (儿化 érhuà). 明儿 is to 明天 what 'tomorra' is to 'tomorrow' - warm, casual, very spoken. You'll hear it constantly in Beijing dialogue and old Lao She novels.
mistakes
The pinyin is míngr (one syllable), not míng-ér. The 儿 is not pronounced as a separate 'er' - it just curls the preceding vowel. Same pattern as 一会儿, 哪儿.
