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/testing/schema_registry_mock.ts

@@ -1,4 +1,5 @@
 import {isPresent} from '../src/facade/lang';
+import {SecurityContext} from '../core_private';
 import {ElementSchemaRegistry} from '../index';
 
 export class MockSchemaRegistry implements ElementSchemaRegistry {
@@ -10,6 +11,10 @@ export class MockSchemaRegistry implements ElementSchemaRegistry {
     return isPresent(result) ? result : true;
   }
 
+  securityContext(tagName: string, property: string): SecurityContext {
+    return SecurityContext.NONE;
+  }
+
   getMappedPropName(attrName: string): string {
     var result = this.attrPropMapping[attrName];
     return isPresent(result) ? result : attrName;