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

tests/html/isolates_test.dart

-#library('IsolatesTest');
-#import('../../pkg/unittest/unittest.dart');
-#import('../../pkg/unittest/html_config.dart');
-#import('dart:html');
-#import('dart:json');
-#import('dart:isolate', prefix:'isolate');
+library IsolatesTest;
+import '../../pkg/unittest/lib/unittest.dart';
+import '../../pkg/unittest/lib/html_config.dart';
+import 'dart:html';
+import 'dart:json';
+import 'dart:isolate' as isolate;
 
 String responseFor(message) => 'response for $message';
 
 void isolateEntry() {
   bool wasThrown = false;
   try {
     window.alert('Test');
   } catch (e) {
     wasThrown = true;
   }
@@ skipping 26 matching lines @@
         port.call(msg2).then((response) {
           guardAsync(() {
             expect(response, equals(responseFor(msg2)));
             callback();
           });
         });
       });
     });
   });
 }