Fix all dart:html strong mode errors except for spurious FINAL_NOT_INITIALIZED_CONSTRUCTOR warnings.

lib/runtime/dart/web_gl.js

Index: lib/runtime/dart/web_gl.js
diff --git a/lib/runtime/dart/web_gl.js b/lib/runtime/dart/web_gl.js
index 67aa8109fe61ad2e2062a14864e28cda2046cc04..21a6dd84d944491ebbafed8739b5b3c9489a38c0 100644
--- a/lib/runtime/dart/web_gl.js
+++ b/lib/runtime/dart/web_gl.js
@@ -387,7 +387,7 @@ dart_library.library('dart/web_gl', null, /* Imports */[
       return this.getBufferParameter(target, pname);
     }
     [dartx.getContextAttributes]() {
-      return html_common.convertNativeToDart_ContextAttributes(this[_getContextAttributes_1]());
+      return dart.as(html_common.convertNativeToDart_ContextAttributes(this[_getContextAttributes_1]()), ContextAttributes);
     }
     [_getContextAttributes_1]() {
       return this.getContextAttributes();