Implement assert() messages

[munificent]

The DEP committee feels Seth's proposal for an optional message to assert() is far enough along to be ready for a real implementation.

the implementation should be gated behind a flag called "assert-message".
The implementations can and should now remove the flag.

This is the main tracking bug. Each implementation's specific bug is below:

• VM (Implement assert() messages behind a flag on VM #24215) - CL https://codereview.chromium.org/2564623003/ (old cl)
• dart2js (Implement assert() messages behind a flag on dart2js #24216) - CL https://codereview.chromium.org/1325843003/
• analyzer (Implement assert() messages behind a flag in analyzer #24217)
• analyzer cli (Exposing option to enable support for asserts w/ messages #25141)
• analysis server (Request to add an option to enable assert + message support in analysis server #24276)
• DDC (Implement assert() messages in dev_compiler #27342)
• dart_style (Implement assert() messages behind a flag in dart_style dart_style#411)

[munificent]

Hey, @jmesserly or @vsmenon, do you want to file a relevant bug for DDC?

[sethladd]

@gbracha can you provide any updates from the DEP's discussion on whether the optional param can be type annotated with dynamic or String ?

Thanks!

[sethladd]

From a chat with @gbracha : The type annotation for the optional parameter will be a String.