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/compiler/src/identifiers.ts

@@ -11,6 +11,7 @@ import {
   ViewEncapsulation,
   TemplateRef
 } from '@angular/core';
+import {SecurityContext} from '../core_private';
 import {
   AppElement,
   AppView,
@@ -199,6 +200,11 @@ export class Identifiers {
     new CompileIdentifierMetadata(
         {name: 'pureProxy10', moduleUrl: VIEW_UTILS_MODULE_URL, runtime: pureProxy10}),
   ];
+  static SecurityContext = new CompileIdentifierMetadata({
+    name: 'SecurityContext',
+    moduleUrl: assetUrl('core', 'security'),
+    runtime: SecurityContext,
+  });
 }
 
 export function identifierToken(identifier: CompileIdentifierMetadata): CompileTokenMetadata {