Make Exception a class, not an interface, and remove the const constructor.

tests/html/indexeddb_2_test.dart

 library IndexedDB1Test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 import 'dart:collection';
 import 'utils.dart';
 
 // Write and re-read Maps: simple Maps; Maps with DAGs; Maps with cycles.
 
 const String DB_NAME = 'Test';
 const String STORE_NAME = 'TEST';
 const int VERSION = 1;
 
 testReadWrite(key, value, check,
               [dbName = DB_NAME,
                storeName = STORE_NAME,
                version = VERSION]) => () {
   var db;
 
   fail(e) {
     guardAsync(() {
-      throw const Exception('IndexedDB failure');
+      throw new Exception('IndexedDB failure');
     });
   }
 
   createObjectStore(db) {
     var store = db.createObjectStore(storeName);
     expect(store, isNotNull);
   }
 
   step2(e) {
     var transaction = db.transaction(storeName, 'readonly');
@@ skipping 91 matching lines @@
   go('test_simple', obj1);
   go('test_DAG', obj2);
   go('test_cycle', obj3);
   go('test_simple_splay', obj4);
   go('const_array_1', const [const [1], const [2]]);
   go('const_array_dag', const [const [1], const [1]]);
   go('array_deferred_copy', [1,2,3, obj3, obj3, 6]);
   go('array_deferred_copy_2', [1,2,3, [4, 5, obj3], [obj3, 6]]);
   go('cyclic_list', cyclic_list);
 }