Make the midpoints test pass. Fixes incorrect Row.reverseSign

packages/cassowary/lib/row.dart

@@ -34,7 +34,10 @@ class _Row {
     cells.remove(symbol);
   }
 
-  void reverseSign() => cells.forEach((s, v) => cells[s] = -v);
+  void reverseSign() {
+    constant = -constant;
+    cells.forEach((s, v) => cells[s] = -v);
+  }
 
   void solveForSymbol(_Symbol symbol) {
     assert(cells.containsKey(symbol));

packages/cassowary/lib/solver.dart

@@ -12,7 +12,7 @@ class Solver {
   final List<_Symbol> _infeasibleRows = new List<_Symbol>();
   final _Row _objective = new _Row(0.0);
   _Row _artificial = new _Row(0.0);
-  int tick = 0;
+  int tick = 1;
 
   /// Attempts to add the constraints in the list to the solver. If it cannot
   /// add any for some reason, a cleanup is attempted so that either all

packages/cassowary/test/cassowary_test.dart

@@ -432,13 +432,13 @@ void main() {
   });
 
   test('midpoints', () {
-    var left = new Param(0.0);
-    var right = new Param(0.0);
-    var mid = new Param(0.0);
+    var left = new Param(0.0)..name = "left";
+    var right = new Param(0.0)..name = "right";
+    var mid = new Param(0.0)..name = "mid";
 
     Solver s = new Solver();
 
-    expect(s.addConstraint((left + right == CM(2.0) * mid) as Constraint),
+    expect(s.addConstraint((right + left == mid * CM(2.0)) as Constraint),
         Result.success);
     expect(s.addConstraint(right - left >= CM(100.0)), Result.success);
     expect(s.addConstraint(left >= CM(0.0)), Result.success);