* fix(compiler): throw an error if variable with the same name is already defined. Closes#6492
* fix(compiler): Clean up formatting for issue #6492
* fix(compiler): throw an error if reference with the same name is already defined.
Closes#6492
Summary:
This adds basic security hooks to Angular 2.
* `SecurityContext` is a private API between core, compiler, and
platform-browser. `SecurityContext` communicates what context a value is used
in across template parser, compiler, and sanitization at runtime.
* `SanitizationService` is the bare bones interface to sanitize values for a
particular context.
* `SchemaElementRegistry.securityContext(tagName, attributeOrPropertyName)`
determines the security context for an attribute or property (it turns out
attributes and properties match for the purposes of sanitization).
Based on these hooks:
* `DomSchemaElementRegistry` decides what sanitization applies in a particular
context.
* `DomSanitizationService` implements `SanitizationService` and adds *Safe
Value*s, i.e. the ability to mark a value as safe and not requiring further
sanitization.
* `url_sanitizer` and `style_sanitizer` sanitize URLs and Styles, respectively
(surprise!).
`DomSanitizationService` is the default implementation bound for browser
applications, in the three contexts (browser rendering, web worker rendering,
server side rendering).
BREAKING CHANGES:
*** SECURITY WARNING ***
Angular 2 Release Candidates do not implement proper contextual escaping yet.
Make sure to correctly escape all values that go into the DOM.
*** SECURITY WARNING ***
Reviewers: IgorMinar
Differential Revision: https://reviews.angular.io/D103
