feat(security): warn users when sanitizing in dev mode.

This should help developers to figure out what's going on when the sanitizer
strips some input.

Fixes #8522.

modules/@angular/platform-browser/src/security/style_sanitizer.ts

@@ -1,3 +1,6 @@
+import {getDOM} from '../dom/dom_adapter';
+import {assertionsEnabled} from '../../src/facade/lang';
+
 /**
  * Regular expression for safe style values.
  *
@@ -44,5 +47,10 @@ function hasBalancedQuotes(value: string) {
 export function sanitizeStyle(value: string): string {
   value = String(value);  // Make sure it's actually a string.
   if (value.match(SAFE_STYLE_VALUE) && hasBalancedQuotes(value)) return value;
+
+  if (assertionsEnabled()) {
+    getDOM().log('WARNING: sanitizing unsafe style value ' + value);
+  }
+
   return 'unsafe';
 }