I third @Bilbosax and @harpwood's suggestion to consider Haxe (language) + OpenFL (Flash-like API) + Starling.
There are some syntax differences as mentioned,
for loops and handling of
static declarations among them, as well as stricter handling of data types (eg: float vs integer, which could be implied with AS3).
Another difference you may encounter is asset handling. Video support is still very limited on most targets although if I'm not mistaken HTML is ok (?), supported audio formats may differ, as well as texture atlas formats and supported compression.