Minor: Add support for priority updates on constraints

packages/cassowary/lib/constraint.dart

@@ -12,4 +12,6 @@ class Constraint {
   double priority = 1000.0;
 
   Constraint(this.expression, this.relation);
+
+  Constraint operator |(double p) => this..priority = p;
 }

packages/cassowary/lib/expression.dart

@@ -6,6 +6,7 @@ part of cassowary;
 
 class Expression extends EquationMember {
   final List<Term> terms;
+
   final double constant;
   double get value => terms.fold(constant, (value, term) => value + term.value);
 

packages/cassowary/test/cassowary_test.dart

@@ -259,4 +259,15 @@ void main() {
     expect(c3.expression.constant, -30.0);
     expect(c3.relation, Relation.lessThanOrEqualTo);
   });
+
+  test('constraint_strength_update', () {
+    var left = new Variable(2.0);
+    var right = new Variable(10.0);
+
+    var c = (right - left >= 200.0) | 750.0;
+    expect(c is Constraint, true);
+    expect(c.expression.terms.length, 2);
+    expect(c.expression.constant, -200.0);
+    expect(c.priority, 750.0);
+  });
 }