Multilink and DerivedProperty should not allow duplicate dependencies

[zepumph]

Discovered while working on phetsims/gravity-force-lab-basics#128.

Right now this "feature" doesn't seem to be used, at least checking via slack.

I also added an assertion like:
assert && assert( _.isEqual( dependencies, _.uniq( dependencies ) ), 'duplicate dependencies' );

to DerivedProperty and Multilink and ran aqua fuzzing with no errors. With this in mind I'm going to commit these changes, and then clean up any issues that may come from this.

[zepumph]

Assigning to myself to look at ct in a few hours, then pass off to review.

[samreid]

https://stackoverflow.com/questions/28461014/using-lodash-to-check-whether-an-array-has-duplicate-values recommends checking _.uniq(a).length !== a.length, that seems like it could avoid some hassles/headaches with _.isEqual corner cases.

[jonathanolson]

Looks like I'm running afoul of this in proportion-playground, but only conditionally:

  PaintChoice.getActiveColorProperty = function( paintChoiceProperty, side ) {
    var dependencies = [ paintChoiceProperty ].concat( PaintChoice.CHOICES.map( function( paintChoice ) {
      return paintChoice.getColorProperty( side );
    } ) );

    return new DerivedProperty( dependencies, function() {
      return paintChoiceProperty.value.getColorProperty( side ).value;
    } );
  };

Should I wrap a _.uniq around it? Noticed in phetsims/proportion-playground#106.

[zepumph]

@samreid implemented above. @jonathanolson that sounds like the right approach.

[zepumph]

Implemented above @jonathanolson. Please review. Feel free to close if all looks good.

[jonathanolson]

Looks good, thanks! Closing as requested.