Rename context2d to context2D.

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.",
         " *",
         " * Example usage:",
         " *",
         " *     var canvas = new CanvasElement(width: 600, height: 600);",
-        " *     var ctx = canvas.context2d;",
+        " *     var ctx = canvas.context2D;",
         " *     ctx.clearRect(0, 0, 600, 600);",
         " *     ctx.save();",
         " *     // Create radial gradient.",
         " *     CanvasGradient gradient = ctx.createRadialGradient(0, 0, 0, 0, 0, 600);",
         " *     gradient.addColorStop(0, '#000');",
         " *     gradient.addColorStop(1, 'rgb(255, 255, 255)');",
         " *     // Assign gradients to fill.",
         " *     ctx.fillStyle = gradient;",
         " *     // Draw a rectangle with a gradient fill.",
         " *     ctx.fillRect(0, 0, 600, 600);",
@@ skipping 24 matching lines @@
     "CanvasPattern": {
       "comment": [
         "/**",
         " * An opaque object representing a pattern of image, canvas, or video.",
         " *",
         " * Created by calling [createPattern] on a [CanvasRenderingContext2D] object.",
         " *",
         " * Example usage:",
         " *",
         " *     var canvas = new CanvasElement(width: 600, height: 600);",
-        " *     var ctx = canvas.context2d;",
+        " *     var ctx = canvas.context2D;",
         " *     var img = new ImageElement();",
         " *     // Image src needs to be loaded before pattern is applied.",
         " *     img.onLoad.listen((event) {",
         " *       // When the image is loaded, create a pattern",
         " *       // from the ImageElement.",
         " *       CanvasPattern pattern = ctx.createPattern(img, 'repeat');",
         " *       ctx.rect(0, 0, canvas.width, canvas.height);",
         " *       ctx.fillStyle = pattern;",
         " *       ctx.fill();",
         " *     });",
@@ skipping 155 matching lines @@
           "   *",
           "   * If the height or width of this canvas element is 0, then 'data:' is returned,",
           "   * representing no data.",
           "   *",
           "   * If the type requested is not 'image/png', and the returned value is",
           "   * 'data:image/png', then the requested type is not supported.",
           "   *",
           "   * Example usage:",
           "   *",
           "   *     CanvasElement canvas = new CanvasElement();",
-          "   *     var ctx = canvas.context2d",
+          "   *     var ctx = canvas.context2D",
           "   *     ..fillStyle = \"rgb(200,0,0)\"",
           "   *     ..fillRect(10, 10, 55, 50);",
           "   *     var dataUrl = canvas.toDataURL(\"image/jpeg\", 0.95);",
           "   *     // The Data Uri would look similar to",
           "   *     // 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA",
           "   *     // AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO",
           "   *     // 9TXL0Y4OHwAAAABJRU5ErkJggg=='",
           "   *     //Create a new image element from the data URI.",
           "   *     var img = new ImageElement();",
           "   *     img.src = dataUrl;",
@@ skipping 355 matching lines @@
           "   *",
           "   * [oldVersion] should match the database's current [version] exactly.",
           "   *",
           "   * * [Database.changeVersion](http://www.w3.org/TR/webdatabase/#dom-database-changeversion) from W3C.",
           "   */"
         ]
       }
     }
   }
 }