Non-private properties now expressly prohibited in trait dfns

:O What?! See Trait/PropertyTest for more information on why this is the
case, at least for now.

lib/Trait.js

@@ -118,6 +118,10 @@ Trait.extend = function( dfn )
         throw Error( "Traits may not define __construct" );
     }
 
+    // traits have property restrictions
+    validateProps( tclass.___$$props$$['public'] );
+    validateProps( tclass.___$$props$$['protected'] );
+
     // invoked to trigger mixin
     TraitType.__mixin = function( dfn, tc, base )
     {
@@ -134,6 +138,33 @@ Trait.extend = function( dfn )
 };
 
 
+/**
+ * Throws error if non-internal property is found within PROPS
+ *
+ * For details and rationale, see the Trait/PropertyTest case.
+ *
+ * @param  {Object}  props  properties to prohibit
+ *
+ * @return  {undefined}
+ */
+function validateProps( props )
+{
+    for ( var f in props )
+    {
+        // ignore internal properties
+        if ( f.substr( 0, 3 ) === '___' )
+        {
+            continue;
+        }
+
+        throw Error(
+            "Cannot define property `" + f + "'; only private " +
+            "properties are permitted within Trait definitions"
+        );
+    }
+}
+
+
 Trait.implement = function()
 {
     var ifaces = arguments;

test/Trait/PropertyTest.js

@@ -0,0 +1,73 @@
+/**
+ * Tests trait properties
+ *
+ *  Copyright (C) 2014 Mike Gerwitz
+ *
+ *  This file is part of GNU ease.js.
+ *
+ *  ease.js is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Or, rather, lack thereof, at least for the time being---this is something
+ * that is complicated by pre-ES5 fallback and, while a solution is
+ * possible, it is not performant in the case of a fallback and would muddy
+ * up ease.js' code.
+ */
+
+require( 'common' ).testCase(
+{
+    caseSetUp: function()
+    {
+        this.Sut = this.require( 'Trait' );
+    },
+
+
+    /**
+     * Since private properties cannot be accessed by anything besides the
+     * trait itself, they cannot interfere with anything else and should be
+     * permitted. Indeed, it would be obsurd to think otherwise, since the
+     * trait should be able to maintain its own local state.
+     */
+    'Private trait properties are permitted': function()
+    {
+        var Sut = this.Sut;
+        this.assertDoesNotThrow( function()
+        {
+            Sut( { 'private _foo': 'bar' } );
+        } );
+    },
+
+
+    /**
+     * See the description at the top of this file. This is something that
+     * may be addressed in future releases.
+     *
+     * Rather than simply ignoring them, we should notify the user that
+     * their code is not going to work as intended and prevent bugs
+     * associated with it.
+     */
+    'Public and protected trait properties are prohibited': function()
+    {
+        var Sut = this.Sut;
+
+        this.assertThrows( function()
+        {
+            Sut( { 'public foo': 'bar' } );
+        } );
+
+        this.assertThrows( function()
+        {
+            Sut( { 'protected foo': 'bar' } );
+        } );
+    },
+} );