Add pattern matching support

[DartBot]

This issue was originally filed by paulp...@gmail.com

---

Another wide-spreading feature in modern programming languages (ES6, CoffeeScript) is non-strict pattern matching.

http://wiki.ecmascript.org/doku.php?id=harmony:destructuring

I'd love to see it's support in dart. Something like:

[a, b] = func()
var data = {a: 1, b: 5}
{a, b} = data
print(a) # 1

[DartBot]

This comment was originally written by jat@google.com

---

Removed Type-Defect label.
Added Type-Enhancement label.

[DartBot]

This comment was originally written by drfibonacci@google.com

---

Added Area-Language, Triaged labels.

[DartBot]

This comment was originally written by drfibonacci@google.com

---

Added Duplicate label.
Marked as being merged into #75.