Use addChainableMethod to get away from __proto__ manipulation.

Helps with #66.

lib/assertion.js

@@ -216,37 +216,23 @@ Object.defineProperty(Assertion.prototype, 'deep',
  * @api public
  */
 
-var an = function () {
-  var assert = function(type) {
-    var obj = flag(this, 'object')
-      , klassStart = type.charAt(0).toUpperCase()
-      , klass = klassStart + type.slice(1)
-      , article = ~[ 'A', 'E', 'I', 'O', 'U' ].indexOf(klassStart) ? 'an ' : 'a ';
-
-    this.assert(
-        '[object ' + klass + ']' === toString.call(obj)
-      , 'expected #{this} to be ' + article + type
-      , 'expected #{this} not to be ' + article + type
-      , '[object ' + klass + ']'
-      , toString.call(obj)
-    );
-
-    return this;
-  };
-
-  assert.__proto__ = this;
-  return assert;
-};
+function an(type) {
+  var obj = flag(this, 'object')
+    , klassStart = type.charAt(0).toUpperCase()
+    , klass = klassStart + type.slice(1)
+    , article = ~[ 'A', 'E', 'I', 'O', 'U' ].indexOf(klassStart) ? 'an ' : 'a ';
 
-Object.defineProperty(Assertion.prototype, 'an',
-  { get: an
-  , configurable: true
-});
+  this.assert(
+      '[object ' + klass + ']' === toString.call(obj)
+    , 'expected #{this} to be ' + article + type
+    , 'expected #{this} not to be ' + article + type
+    , '[object ' + klass + ']'
+    , toString.call(obj)
+  );
+}
 
-Object.defineProperty(Assertion.prototype, 'a',
-  { get: an
-  , configurable: true
-});
+Assertion.addChainableMethod('an', an);
+Assertion.addChainableMethod('a', an);
 
 /**
  * ### .include(value)
@@ -266,32 +252,20 @@ Object.defineProperty(Assertion.prototype, 'a',
  * @api public
  */
 
-var include = function () {
+function includeChainingBehavior() {
   flag(this, 'contains', true);
+}
 
-  var assert = function(val) {
-    var obj = flag(this, 'object')
-    this.assert(
-        ~obj.indexOf(val)
-      , 'expected #{this} to include ' + util.inspect(val)
-      , 'expected #{this} to not include ' + util.inspect(val));
-
-    return this;
-  };
-
-  assert.__proto__ = this;
-  return assert;
-};
-
-Object.defineProperty(Assertion.prototype, 'contain',
-  { get: include
-  , configurable: true
-});
+function include(val) {
+  var obj = flag(this, 'object')
+  this.assert(
+      ~obj.indexOf(val)
+    , 'expected #{this} to include ' + util.inspect(val)
+    , 'expected #{this} to not include ' + util.inspect(val));
+}
 
-Object.defineProperty(Assertion.prototype, 'include',
-  { get: include
-  , configurable: true
-});
+Assertion.addChainableMethod('include', include, includeChainingBehavior);
+Assertion.addChainableMethod('contain', include, includeChainingBehavior);
 
 /**
  * ### .ok