refactor($parse): remove Angular expression sandbox

The angular expression parser (`$parse`) attempts to sandbox expressions
to prevent unrestricted access to the global context.

While the sandbox was not on the frontline of the security defense,
developers kept relying upon it as a security feature even though it was
always possible to access arbitrary JavaScript code if a malicious user
could control the content of Angular templates in applications.

This commit removes this sandbox, which has the following benefits:

* it sends a clear message to developers that they should not rely on
the sandbox to prevent XSS attacks; that they must prevent control of
expression and templates instead.
* it allows performance and size improvements in the core Angular 1
library.
* it simplifies maintenance and provides opportunities to make the
parser more capable.

Please see the [Sandbox Removal Blog Post](http://angularjs.blogspot.com/2016/09/angular-16-expression-sandbox-removal.html)
for more detail on what you should do to ensure that your application is
secure.

Closes #15094

Author: petebacondarwin

docs/content/error/$parse/isecaf.ngdoc

@@ -113,11 +113,11 @@ You can try evaluating different expressions here:
 Angular does not use JavaScript's `eval()` to evaluate expressions. Instead Angular's
 {@link ng.$parse $parse} service processes these expressions.
 
-Angular expressions do not have access to global variables like `window`, `document` or `location`.
+Angular expressions do not have direct access to global variables like `window`, `document` or `location`.
 This restriction is intentional. It prevents accidental access to the global state – a common source of subtle bugs.
 
-Instead use services like `$window` and `$location` in functions called from expressions. Such services
-provide mockable access to globals.
+Instead use services like `$window` and `$location` in functions on controllers, which are then called from expressions.
+Such services provide mockable access to globals.
 
 It is possible to access the context object using the identifier `this` and the locals object using the
 identifier `$locals`.

docs/content/error/$parse/isecdom.ngdoc

@@ -30,42 +30,55 @@ so keeping to AngularJS standards is not just a functionality issue, it's also c
 facilitate rapid security updates.
 
 
-## Expression Sandboxing
-
-AngularJS's expressions are sandboxed not for security reasons, but instead to maintain a proper
-separation of application responsibilities. For example, access to `window` is disallowed
-because it makes it easy to introduce brittle global state into your application.
-
-However, this sandbox is not intended to stop attackers who can edit the template before it's
-processed by Angular. It may be possible to run arbitrary JavaScript inside double-curly bindings
-if an attacker can modify them.
-
-But if an attacker can change arbitrary HTML templates, there's nothing stopping them from doing:
-
-```html
-<script>somethingEvil();</script>
-```
-
-**It's better to design your application in such a way that users cannot change client-side templates.**
-
-For instance:
+## Angular Templates and Expressions
+
+**If an attacker has access to control Angular templates or expressions, they can exploit an Angular application
+via an XSS attack, regardless of the version.**
+
+There are a number of ways that templates and expressions can be controlled:
+
+* **Generating Angular templates on the server containing user-provided content**. This is the most common pitfall
+  where you are generating HTML via some server-side engine such as PHP, Java or ASP.NET.
+* **Passing an expression generated from user-provided content in calls to the following methods on a {@link scope scope}**:
+  * `$watch(userContent, ...)`
+  * `$watchGroup(userContent, ...)`
+  * `$watchCollection(userContent, ...)`
+  * `$eval(userContent)`
+  * `$evalAsync(userContent)`
+  * `$apply(userContent)`
+  * `$applyAsync(userContent)`
+* **Passing an expression generated from user-provided content in calls to services that parse expressions**:
+  * `$compile(userContent)`
+  * `$parse(userContent)`
+  * `$interpolate(userContent)`
+* **Passing an expression generated from user provided content as a predicate to `orderBy` pipe**:
+  `{{ value | orderBy : userContent }}`
+
+### Sandbox removal
+Each version of Angular 1 up to, but not including 1.6, contained an expression sandbox, which reduced the surface area of
+the vulnerability but never removed it. **In Angular 1.6 we removed this sandbox as developers kept relying upon it as a security
+feature even though it was always possible to access arbitrary JavaScript code if one could control the Angular templates
+or expressions of applications.**
+
+Control of the Angular templates makes applications vulnerable even if there was a completely secure sandbox:
+* https://ryhanson.com/stealing-session-tokens-on-plunker-with-an-angular-expression-injection/ in this blog post the author shows
+  a (now closed) vulnerability in the Plunker application due to server-side rendering inside an Angular template.
+* https://ryhanson.com/angular-expression-injection-walkthrough/ in this blog post the author describes an attack, which does not
+  rely upon an expression sandbox bypass, that can be made because the sample application is rendering a template on the server that
+  contains user entered content.
+
+**It's best to design your application in such a way that users cannot change client-side templates.**
 
 * Do not mix client and server templates
 * Do not use user input to generate templates dynamically
-* Do not run user input through `$scope.$eval`
+* Do not run user input through `$scope.$eval` (or any of the other expression parsing functions listed above)
 * Consider using {@link ng.directive:ngCsp CSP} (but don't rely only on CSP)
 
+**You can use suitably sanitized server-side templating to dynamically generate CSS, URLs, etc, but not for generating templates that are
+bootstrapped/compiled by Angular.**
 
-### Mixing client-side and server-side templates
-
-In general, we recommend against this because it can create unintended XSS vectors.
-
-However, it's ok to mix server-side templating in the bootstrap template (`index.html`) as long
-as user input cannot be used on the server to output html that would then be processed by Angular
-in a way that would allow for arbitrary code execution.
-
-**For instance, you can use server-side templating to dynamically generate CSS, URLs, etc, but not
-for generating templates that are bootstrapped/compiled by Angular.**
+**If you must continue to allow user-provided content in an Angular template then the safest option is to ensure that it is only
+present in the part of the template that is made inert via the {@link ngNonBindable} directive.**
 
 
 ## HTTP Requests