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

tests/html/webgl_1_test.dart

-#library('WebGL1Test');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
+library WebGL1Test;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
 
 // Test that WebGL is present in dart:html API
 
 main() {
   useHtmlConfiguration();
 
   test('simple', () {
       var canvas = document.createElement("canvas");
       var gl = canvas.getContext("experimental-webgl");
       var shader = gl.createShader(WebGLRenderingContext.VERTEX_SHADER);
       gl.shaderSource(shader, "void main() { }");
       gl.compileShader(shader);
       var success =
           gl.getShaderParameter(shader, WebGLRenderingContext.COMPILE_STATUS);
       expect(success, isTrue);
   });
 }