fix($compile): isolate and non-isolate directives on the same element

A non-isolate directive should not get the isolate scope of an isolate directive on the same element.

Paired with Tobias.

BREAKING CHANGE: Directives without isolate scope do not get the isolate scope from an isolate directive on the same element. If your code depends on this behavior (non-isolate directive needs to access state from within the isolate scope), change the isolate directive to use scope locals to pass these explicitly.

// before
<input ng-model="value" ng-isolate>

.directive('ngIsolate', function() {
  return {
    scope: {},
    template: '{{value}}'
  };
});

// after
<input ng-model="value" ng-isolate>

.directive('ngIsolate', function() {
  return {
    scope: {value: '=ngModel'},
    template: '{{value}}
  };
});