Implement assert() messages in dev_compiler

[jmesserly]

From @vsmenon on August 26, 2015 21:38

This adds optional messages to an assert.

See DEP: https://github.com/sethladd/dep_assert_with_optional_message/blob/master/proposal.md

Main bug: #24213

Copied from original issue: dart-archive/dev_compiler#303

[jmesserly]

behind a flag? Aren't we entirely behind a flag? ;)

[jmesserly]

(experimental language flags to an experimental compiler, we must go deeper :) )

edit: in seriousness though, I think analyzer's flag will protect us, since they're the ones doing the parsing.

[jmesserly]

oh shoot, i realized we're blocked on analyzer: #24217

[vsmenon]

@bwilkerson can you assign this back to me when #24217 is closed? The DDC part should be really easy.

[dgrove]

@lrhn Can you provide an update?