Fix for getContextAttributes which wraps bare dictionary into a Dart type.

sdk/lib/web_gl/dartium/web_gl_dartium.dart

Index: sdk/lib/web_gl/dartium/web_gl_dartium.dart
diff --git a/sdk/lib/web_gl/dartium/web_gl_dartium.dart b/sdk/lib/web_gl/dartium/web_gl_dartium.dart
index e09051ca3ccd4753521d4021f5cc226de8a9e736..a3c428694fce42abce05dc5d89edd0cdaa768513 100644
--- a/sdk/lib/web_gl/dartium/web_gl_dartium.dart
+++ b/sdk/lib/web_gl/dartium/web_gl_dartium.dart
@@ -492,6 +492,28 @@ class CompressedTextureS3TC extends NativeFieldWrapperClass2 {
 
 
 @DocsEditable()
+/**
+ * The properties of a WebGL rendering context.
+ *
+ * If [alpha] is `true`, then the context has an alpha channel.
+ *
+ * If [antialias] is `true`, then antialiasing is performed by the browser, but
+ * only if the browser's implementation of WebGL supports antialiasing.
+ *
+ * If [depth] is `true`, then the context has a depth buffer of at least 16
+ * bits.
+ *
+ * If [premultipliedAlpha] is `true`, then the context's colors are assumed to
+ * be premultiplied. This means that color values are assumed to have  been
+ * multiplied by their alpha values. If [alpha] is `false`, then this flag is
+ * ignored.
+ *
+ * If [preserveDrawingBuffer] is `false`, then all contents of the context are
+ * cleared. If `true`, then all values will remain until changed or cleared.
+ *
+ * If [stencil] is `true`, then the context has a stencil buffer of at least 8
+ * bits.
+ */
 @DomName('WebGLContextAttributes')
 @Unstable()
 class ContextAttributes extends NativeFieldWrapperClass2 {