Refactor Future constructors.

tests/html/websql_test.dart

 library WebDBTest;
 import 'dart:async';
 import 'dart:html';
 import 'dart:web_sql';
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 
 void fail(message) {
   guardAsync(() {
       expect(false, isTrue, reason: message);
@@ skipping 92 matching lines @@
     test('unsupported throws', () {
       var expectation = SqlDatabase.supported ? returnsNormally : throws;
       expect(() {
         window.openDatabase('test_db', '1.0', 'test_db', 1024 * 1024);
       }, expectation);
 
     });
     test('Web Database', () {
       // Skip if not supported.
       if (!SqlDatabase.supported) {
-        return new Future.immediate(null);
+        return new Future.value();
       }
 
       final tableName = 'test_table';
       final columnName = 'test_data';
 
       final db = window.openDatabase('test_db', '1.0', 'test_db', 1024 * 1024);
 
       expect(db, isNotNull, reason: 'Unable to open database');
 
       var tx;
@@ skipping 14 matching lines @@
         var row = resultSet.rows.item(0);
         expect(row.containsKey(columnName), isTrue);
         expect(row[columnName], 'Some text data');
         expect(resultSet.rows[0], row);
       }).then((_) {
         return dropTable(tx, tableName);
       });
     });
   });
 }