Checked exceptions

[yichaosun]

Change Intent

CheckException can force engineer to handle exception,

Justification

I worked with C++, Java, and finally, found that check Exception is necessary and more engineering

Impact

to make programs more strong，no exception throw without handle

Mitigation

likee no empty check, the programmer must handle exist exception ,and even use throws key word

[lrhn]

This is a duplicate of dart-lang/language#984
There are currently no plans to add checked exceptions to Dart, and not current plans to change that.