feat: security implementation in Angular 2.

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

modules/@angular/integration_test/public_api_spec.ts

@@ -362,6 +362,7 @@ var PLATFORM_BROWSER: string[] = [
   'BROWSER_APP_STATIC_PROVIDERS',
   'BROWSER_PROVIDERS',
   'BROWSER_APP_COMMON_PROVIDERS',
+  'BROWSER_SANITIZATION_PROVIDERS',
   'DOCUMENT',
   'ELEMENT_PROBE_PROVIDERS',
   'DomEventsPlugin',
@@ -375,6 +376,8 @@ var PLATFORM_BROWSER: string[] = [
   'BrowserPlatformLocation',
   'AngularEntrypoint:dart',
   'By',
+  'DomSanitizationService',
+  'SecurityContext',
   'Title',
   'disableDebugTools',
   'enableDebugTools'