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

tools/dom/docs/docs.json

 {
   "dart.dom.html": {
     "CanvasGradient": {
       "comment": [
         "/**",
         " * An opaque canvas object representing a gradient.",
         " *",
         " * Created by calling [createLinearGradient] or [createRadialGradient] on a",
         " * [CanvasRenderingContext2D] object.",
         " *",
@@ skipping 512 matching lines @@
           "   * This particular stream is special in that it only allows one listener to a",
           "   * given stream. Converting the returned Stream [asBroadcast] will likely ruin",
           "   * the soft-real-time properties which which these events are fired and can",
           "   * be processed.",
           "   */"
         ]
       }
     }
   },
   "dart.dom.web_gl": {
+    "WebGLContextAttributes": {
+      "comment": [
+        "/**",
+        " * 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.",
+        " */"
+      ]
+    },
     "WebGLRenderingContext": {
       "members": {
         "bufferData": [
           "/**",
           "   * Buffers the specified data.",
           "   *",
           "   * This specific method is provided for WebGL API compatibility reasons, but",
           "   * it is highly recommended that you use [bufferDataTyped] or [bufferByteData]",
           "   * depending on your purposes.",
           "   */"
@@ skipping 44 matching lines @@
           "   *",
           "   * [oldVersion] should match the database's current [version] exactly.",
           "   *",
           "   * * [Database.changeVersion](http://www.w3.org/TR/webdatabase/#dom-database-changeversion) from W3C.",
           "   */"
         ]
       }
     }
   }
 }