Minor: Add result types for known failure cases

packages/cassowary/lib/cassowary.dart

@@ -14,3 +14,4 @@ part 'solver.dart';
 part 'symbol.dart';
 part 'row.dart';
 part 'utils.dart';
+part 'result.dart';

packages/cassowary/lib/result.dart

+// Copyright (c) 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+part of cassowary;
+
+class Result {
+  final bool error;
+  final String message;
+
+  const Result(this.message, this.error);
+
+  static final Result success = const Result("Success", false);
+  static final Result unimplemented = const Result("Unimplemented", true);
+  static final Result duplicateConstraint =
+      const Result("Duplicate Constraint", true);
+  static final Result unsatisfiableConstraint =
+      const Result("Unsatisfiable Constraint", true);
+  static final Result unknownConstraint =
+      const Result("Unknown Constraint", true);
+  static final Result duplicateEditVariable =
+      const Result("Duplicate Edit Variable", true);
+  static final Result badRequiredStrength =
+      const Result("Bad Required Strength", true);
+  static final Result unknownEditVariable =
+      const Result("Unknown Edit Variable", true);
+  static final Result internalSolverError =
+      const Result("Internal Solver Error", true);
+}

packages/cassowary/lib/solver.dart

@@ -13,32 +13,32 @@ class Solver {
   final Row _objective = new Row();
   final Row _artificial = new Row();
 
-  bool addConstraint(Constraint c) {
-    return false;
+  Result addConstraint(Constraint c) {
+    return Result.unimplemented;
   }
 
-  bool removeContraint(Constraint c) {
-    return false;
+  Result removeContraint(Constraint c) {
+    return Result.unimplemented;
   }
 
-  bool hasConstraint(Constraint c) {
-    return false;
+  Result hasConstraint(Constraint c) {
+    return Result.unimplemented;
   }
 
-  bool addEditVariable(Variable v, double priority) {
-    return false;
+  Result addEditVariable(Variable v, double priority) {
+    return Result.unimplemented;
   }
 
-  bool removeEditVariable(Variable v) {
-    return false;
+  Result removeEditVariable(Variable v) {
+    return Result.unimplemented;
   }
 
-  bool hasEditVariable(Variable v) {
-    return false;
+  Result hasEditVariable(Variable v) {
+    return Result.unimplemented;
   }
 
-  bool suggestVariable(Variable v, double value) {
-    return false;
+  Result suggestVariable(Variable v, double value) {
+    return Result.unimplemented;
   }
 
   void updateVariable() {}