Move unittest from lib to pkg.

tests/html/webgl_1_test.dart

 #library('WebGL1Test');
-#import('../../lib/unittest/unittest.dart');
-#import('../../lib/unittest/html_config.dart');
+#import('../../pkg/unittest/unittest.dart');
+#import('../../pkg/unittest/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.isTrue(success);
   });
 }