Add Existential operator support

[DartBot]

This issue was originally filed by demis.bell...@gmail.com

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It would be nice if we could prevent null ref errors by adding Existential operator support.

e.g. Instead of
   if(foo != null && foo.bar != null && foo.bar.baz != null) {
      print (foo.bar.baz.monkey);
   }

you have

   print foo?.bar?.baz?.monkey

[DartBot]

This comment was originally written by dha...@google.com

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Are you suggesting that the ?. operator skip the entirety of the current statement if it attempts to dereference null? Otherwise those two snippets have different semantics.

[munificent]

Added Duplicate label.
Marked as being merged into #41.

[DartBot]

This comment was originally written by demis.bel...@gmail.com

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Apologies, I copied the example from this previous thread:
https://groups.google.com/a/dartlang.org/forum/?fromgroups=#!topic/misc/eqfR585eDVY

Here's a clearer example in CoffeeScript:

doSomethingWithPostcode = (x) => alert(x)
 
doSomethingWithPostcode(customer?.address?.postcode) #this will still call with undefind
 
doSomethingWithPostcode(customer.address.postcode) if customer?.address?.postcode #this wont call the method at all

The generated JavaScript when pasted in http://coffeescript.org/ becomes:

var doSomethingWithPostcode, _ref, _ref1,
  _this = this;
 
doSomethingWithPostcode = function(x) {
  return alert(x);
};
 
doSomethingWithPostcode(typeof customer !== "undefined" && customer !== null ? (_ref = customer.address) != null ? _ref.postcode : void 0 : void 0);
 
if (typeof customer !== "undefined" && customer !== null ? (_ref1 = customer.address) != null ? _ref1.postcode : void 0 : void 0) {
  doSomethingWithPostcode(customer.address.postcode);
}