he/him 
In TC39 we discussed reserving : after a declarator so implementations could not de-facto standardize incompatible type annotations. We knew this would oblige us to standardize type anno/expr syntax at some point. We didn't figure on Flow using ambiguous expression syntax! @awbjs
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What if we just make JS parsing rules stricter if you want to use JSX and Flow/Typescript. Like requiring spaces around operators. I know we call them supersets of javascript, but maybe they don’t need to be strict supersets.
That's not really necessary. Ambiguous syntax doesn't mean spec incompatible. Type arguments on calls are the only syntax extensions that cause otherwise valid regular JS to be parsed differently.