Eliminate support of legacy type Dyamic

tests/html/instance_of_test.dart

 library InstanceOfTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
   CanvasElement canvas;
 
   canvas = new Element.tag('canvas');
   canvas.attributes['width'] = 100;
@@ skipping 25 matching lines @@
 
     CanvasRenderingContext2D context = canvas.getContext('2d');
     expect(context, isCanvasRenderingContext);
     expect(context, isCanvasRenderingContext2D);
     expect(context, isNot(isElement));
     expect(context, isNot(isCanvasElement));
     expect(context, isNot(isImageData));
     // expect(context, isNot(isCanvasPixelArray));
 
     // FIXME(b/5286633): Interface injection type check workaround.
-    var image = context.createImageData(canvas.width as Dynamic,
-                                        canvas.height as Dynamic);
+    var image = context.createImageData(canvas.width as dynamic,
+                                        canvas.height as dynamic);
     expect(image, isNot(isCanvasRenderingContext));
     expect(image, isNot(isCanvasRenderingContext2D));
     expect(image, isNot(isElement));
     expect(image, isNot(isCanvasElement));
     expect(image, isImageData);
     // expect(image, isNot(isCanvasPixelArray));
 
     // Include CanvasPixelArray since constructor and prototype are not
     // available until one is created.
     var bytes = image.data;
     expect(bytes, isNot(isCanvasRenderingContext));
     expect(bytes, isNot(isCanvasRenderingContext2D));
     expect(bytes, isNot(isElement));
     expect(bytes, isNot(isCanvasElement));
     expect(bytes, isNot(isImageData));
     expect(bytes, isUint8ClampedArray);
 
     // FIXME: Ensure this is an SpanElement when we next update
     // WebKit IDL.
     var span = new Element.tag('span');
     expect(span, isElement);
   });
 }