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

lib/runtime/dart/web_gl.js

 dart_library.library('dart/web_gl', null, /* Imports */[
   'dart/_runtime',
   'dart/core',
   'dart/html_common',
   'dart/html',
   'dart/_interceptors',
   'dart/typed_data',
   'dart/_metadata',
   'dart/_js_helper'
 ], /* Lazy imports */[
@@ skipping 369 matching lines @@
     [dartx.getAttachedShaders](program) {
       return this.getAttachedShaders(program);
     }
     [dartx.getAttribLocation](program, name) {
       return this.getAttribLocation(program, name);
     }
     [dartx.getBufferParameter](target, pname) {
       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();
     }
     [dartx.getError]() {
       return this.getError();
     }
     [dartx.getExtension](name) {
       return this.getExtension(name);
     }
@@ skipping 1986 matching lines @@
   exports.OesTextureHalfFloatLinear = OesTextureHalfFloatLinear;
   exports.OesVertexArrayObject = OesVertexArrayObject;
   exports.Program = Program;
   exports.Renderbuffer = Renderbuffer;
   exports.Shader = Shader;
   exports.ShaderPrecisionFormat = ShaderPrecisionFormat;
   exports.Texture = Texture;
   exports.UniformLocation = UniformLocation;
   exports.VertexArrayObject = VertexArrayObject;
 });