fix($parse): mark constant unary minus expressions as constant

Previously, constant numbers with a unary minus sign were not treated as constants. This fix corrects
this behaviour, and may provide a small performance boost for certain applications, due to constant
watches being automatically unregistered after their first listener call.

Closes #6932

src/ng/parse.js

@@ -393,7 +393,11 @@ var Parser = function (lexer, $filter, options) {
   this.options = options;
 };
 
-Parser.ZERO = function () { return 0; };
+Parser.ZERO = extend(function () {
+  return 0;
+}, {
+  constant: true
+});
 
 Parser.prototype = {
   constructor: Parser,

test/ng/parseSpec.js

@@ -1031,6 +1031,7 @@ describe('parser', function() {
 
           it('should mark complex expressions involving constant values as constant', inject(function($parse) {
             expect($parse('!true').constant).toBe(true);
+            expect($parse('-42').constant).toBe(true);
             expect($parse('1 - 1').constant).toBe(true);
             expect($parse('"foo" + "bar"').constant).toBe(true);
             expect($parse('5 != null').constant).toBe(true);