fix($compile): do not initialize optional '&' binding if attribute not specified

BREAKING CHANGE:

Previously, '&' expressions would always set up a function in the isolate scope. Now, if the binding
is marked as optional and the attribute is not specified, no function will be added to the isolate scope.

Closes #6404
Closes #9216

Author: caitp

src/ng/compile.js

@@ -2576,7 +2576,14 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
             break;
 
           case '&':
+            // Don't assign Object.prototype method to scope
+            if (!attrs.hasOwnProperty(attrName) && optional) break;
+
             parentGet = $parse(attrs[attrName]);
+
+            // Don't assign noop to destination if expression is not valid
+            if (parentGet === noop && optional) break;
+
             destination[scopeName] = function(locals) {
               return parentGet(scope, locals);
             };

src/ng/parse.js

@@ -1079,7 +1079,7 @@ function $ParseProvider() {
           return addInterceptor(exp, interceptorFn);
 
         default:
-          return addInterceptor(noop, interceptorFn);
+          return noop;
       }
     };
 

test/ng/compileSpec.js

@@ -3084,7 +3084,9 @@ describe('$compile', function() {
             colref: '=*',
             colrefAlias: '=* colref',
             expr: '&',
-            exprAlias: '&expr'
+            optExpr: '&?',
+            exprAlias: '&expr',
+            constructor: '&?'
           },
           link: function(scope) {
             componentScope = scope;
@@ -3224,6 +3226,38 @@ describe('$compile', function() {
     });
 
 
+    it('should not initialize scope value if optional expression binding is not passed', inject(function($compile) {
+      compile('<div my-component></div>');
+      var isolateScope = element.isolateScope();
+      expect(isolateScope.optExpr).toBeUndefined();
+    }));
+
+
+    it('should not initialize scope value if optional expression binding with Object.prototype name is not passed', inject(function($compile) {
+      compile('<div my-component></div>');
+      var isolateScope = element.isolateScope();
+      expect(isolateScope.constructor).toBe($rootScope.constructor);
+    }));
+
+
+    it('should initialize scope value if optional expression binding is passed', inject(function($compile) {
+      compile('<div my-component opt-expr="value = \'did!\'"></div>');
+      var isolateScope = element.isolateScope();
+      expect(typeof isolateScope.optExpr).toBe('function');
+      expect(isolateScope.optExpr()).toBe('did!');
+      expect($rootScope.value).toBe('did!');
+    }));
+
+
+    it('should initialize scope value if optional expression binding with Object.prototype name is passed', inject(function($compile) {
+      compile('<div my-component constructor="value = \'did!\'"></div>');
+      var isolateScope = element.isolateScope();
+      expect(typeof isolateScope.constructor).toBe('function');
+      expect(isolateScope.constructor()).toBe('did!');
+      expect($rootScope.value).toBe('did!');
+    }));
+
+
     describe('bind-once', function() {
 
       function countWatches(scope) {