Minor: Add stubs for the symbol and solver

packages/cassowary/lib/cassowary.dart

@@ -11,3 +11,4 @@ part 'variable.dart';
 part 'equation_member.dart';
 part 'constant_member.dart';
 part 'solver.dart';
+part 'symbol.dart';

packages/cassowary/lib/constant_member.dart

@@ -6,6 +6,7 @@ part of cassowary;
 
 class ConstantMember extends EquationMember {
   double value = 0.0;
+
   ConstantMember(this.value);
 
   Expression asExpression() => new Expression([], this.value);

packages/cassowary/lib/constraint.dart

@@ -9,9 +9,19 @@ enum Relation { equalTo, lessThanOrEqualTo, greaterThanOrEqualTo, }
 class Constraint {
   final Relation relation;
   final Expression expression;
-  double priority = 1000.0;
+  final bool required;
 
-  Constraint(this.expression, this.relation);
+  static const double requiredPriority = 1000.0;
+  double _priority = requiredPriority - 1.0;
+
+  Constraint(this.expression, this.relation) : this.required = false;
+  Constraint.Required(this.expression, this.relation) : this.required = true {
+    this.priority = requiredPriority;
+  }
+
+  double get priority => required ? requiredPriority : _priority;
+  set priority(double p) => _priority =
+      required ? requiredPriority : p.clamp(0.0, requiredPriority - 1.0);
 
   Constraint operator |(double p) => this..priority = p;
 }

packages/cassowary/lib/solver.dart

@@ -13,5 +13,27 @@ class Solver {
     return false;
   }
 
+  bool hasConstraint(Constraint c) {
+    return false;
+  }
+
+  bool addEditVariable(Variable v, double priority) {
+    return false;
+  }
+
+  bool removeEditVariable(Variable v) {
+    return false;
+  }
+
+  bool hasEditVariable(Variable v) {
+    return false;
+  }
+
+  bool suggestVariable(Variable v, double value) {
+    return false;
+  }
+
+  void updateVariable() {}
+
   Solver operator <<(Constraint c) => this..addConstraint(c);
 }

packages/cassowary/lib/symbol.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;
+
+enum SymbolType { invalid, external, slack, error, dummy, }
+
+class Symbol {
+  SymbolType type;
+
+  Symbol(this.type);
+}

packages/cassowary/test/cassowary_test.dart

@@ -280,6 +280,8 @@ void main() {
     var c1 = right - left >= CM(200.0);
     var c2 = right + left >= CM(0.0);
 
+    expect((right >= left) is Constraint, true);
+
     // TODO: Add assertions for this
     s << c1 << c2;
   });