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

tests/html/canvas_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';
 
 main() {
   CanvasElement canvas;
   CanvasRenderingContext2D context;
   int width = 100;
   int height = 100;
 
   canvas = new Element.tag('canvas');
   canvas.attributes['width'] = width;
   canvas.attributes['height'] = height;
@@ skipping 16 matching lines @@
   });
 }
 
 void checkPixel(Uint8ClampedArray data, int offset, List<int> rgba)
 {
   offset *= 4;
   for (var i = 0; i < 4; ++i) {
     expect(data[offset + i], equals(rgba[i]));
   }
 }