Restructure pkg/unittest and pkg/webdriver to follow the pub conventions.

tests/html/canvas_pixel_array_type_alias_test.dart

-#library('CanvasTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library CanvasTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 // We have aliased the legacy type CanvasPixelArray with the new type
 // Uint8ClampedArray by mapping the CanvasPixelArray type tag to
 // Uint8ClampedArray.  It is not a perfect match since CanvasPixelArray is
 // missing the ArrayBufferView members.  These should appear to be null.
 
 Object confuseType(x) => [1, x, [x], 's'] [1];
 
 main() {
   CanvasElement canvas;
@@ skipping 28 matching lines @@
   });
 }
 
 void checkPixel(Uint8ClampedArray data, int offset, List<int> rgba)
 {
   offset *= 4;
   for (var i = 0; i < 4; ++i) {
     expect(rgba[i], equals(data[offset + i]));
   }
 }