fix($parse): handle constants as one-time binding expressions

Handle constant expressions as one-time binding expressions.
Avoids the infinite digest from
https://github.com/angular/angular.js/pull/7960/files#r14136938

Closes #7970

Author: lgalfaso

src/ng/parse.js

@@ -1027,11 +1027,7 @@ function $ParseProvider() {
             cache[exp] = parsedExpression;
           }
 
-          if (parsedExpression.constant) {
-            parsedExpression.$$unwatch = true;
-          }
-
-          return oneTime ? oneTimeWrapper(parsedExpression) : parsedExpression;
+          return oneTime || parsedExpression.constant ? oneTimeWrapper(parsedExpression) : parsedExpression;
 
         case 'function':
           return exp;
@@ -1050,7 +1046,7 @@ function $ParseProvider() {
 
         function oneTimeParseFn(self, locals) {
           if (!stable) {
-            lastValue = expression(self, locals);
+            lastValue = expression.constant && lastValue ? lastValue : expression(self, locals);
             oneTimeParseFn.$$unwatch = isDefined(lastValue);
             if (oneTimeParseFn.$$unwatch && self && self.$$postDigestQueue) {
               self.$$postDigestQueue.push(function () {

test/ng/rootScopeSpec.js

@@ -114,6 +114,15 @@ describe('Scope', function() {
       expect($rootScope.$$watchers.length).toEqual(0);
     }));
 
+    it('should not keep constant literals on the watch queue', inject(function($rootScope) {
+      $rootScope.$watch('[]', function() {});
+      $rootScope.$watch('{}', function() {});
+      expect($rootScope.$$watchers.length).toEqual(2);
+      $rootScope.$digest();
+
+      expect($rootScope.$$watchers.length).toEqual(0);
+    }));
+
     it('should clean up stable watches on the watch queue', inject(function($rootScope, $parse) {
       $rootScope.$watch($parse('::foo'), function() {});
       expect($rootScope.$$watchers.length).toEqual(1);