Account for the result of optimization when adding constraints

packages/cassowary/lib/solver.dart

@@ -46,9 +46,7 @@ class Solver {
 
     _constraints[constraint] = tag;
 
-    _optimizeObjectiveRow(_objective);
-
-    return Result.success;
+    return _optimizeObjectiveRow(_objective);
   }
 
   Result removeContraint(Constraint c) {

packages/cassowary/test/cassowary_test.dart

@@ -282,8 +282,7 @@ void main() {
 
     expect((right >= left) is Constraint, true);
 
-    // TODO: Add assertions for this
-    s << c1 << c2;
+    expect(s.addConstraint(c1), Result.success);
   });
 
   test('constraint_complex', () {