Add "ensure" and "invariant" checking for contract based assertions and static analysis

[DartBot]

This issue was originally filed by [email protected]

---

I am a big fan of the Code Contracts (http://msdn.microsoft.com/en-us/library/dd264808(v=vs.110).aspx) functionality that Microsoft have added to .NET and would love to see something similar in Dart.

I would guess a first step would be to in addition to "assert" also add "ensure" and "invariant" test in runtime and later on also allow it to help static analysis.

[kasperl]

Added Area-Library, Triaged labels.

[lrhn]

Removed Type-Defect, Area-Library labels.
Added Type-Enhancement, Area-Language labels.

[DartBot]

This comment was originally written by [email protected]

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To illustrate how one would do this without language support:

class Foo
{
    Foo()
    {
         checkInvariants();
         assert(/* requirement */);

         try
         {
             /* implementation /
         }
         finally
         {
              assert(/ ensures */);
              checkInvariants();
         }
    }

    void checkInvariants()
    {
         assert(/* invariant */);
    }
}

Which gives a lot of code clutter...

[DartBot]

This comment was originally written by [email protected]

---

Instead of something like:

class Foo
{
    Foo()
    {
        requires
        {
            /* requirement /
        }
        
        ensures
        {
            / ensures /
        }
        
         / implementation /
    }
    
    invariants
    {
         / invariant */;
    }
}

[gbracha]

Added Duplicate label.
Marked as being merged into #3049.