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

tests/html/indexeddb_1_test.dart

 library IndexedDB1Test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 const String DB_NAME = 'Test';
 const String STORE_NAME = 'TEST';
 const int VERSION = 1;
 
 testReadWrite(key, value, matcher,
               [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 55 matching lines @@
 };
 
 testReadWriteTyped(key, value, matcher,
                    [dbName = DB_NAME,
                     storeName = STORE_NAME,
                     version = VERSION]) => () {
   IDBDatabase db;
 
   fail(e) {
     guardAsync(() {
-      throw const Exception('IndexedDB failure');
+      throw new Exception('IndexedDB failure');
     });
   }
 
   createObjectStore(db) {
     IDBObjectStore store = db.createObjectStore(storeName);
     expect(store, isNotNull);
   }
 
   step2(e) {
     IDBTransaction transaction = db.transaction(storeName, 'readonly');
@@ skipping 67 matching lines @@
   test('test3', testReadWriteTyped(123, [1, 2, 3], equals([1, 2, 3])));
   test('test4', testReadWriteTyped(123, [2, 3, 4], equals([2, 3, 4])));
 }
 
 main() {
   useHtmlConfiguration();
 
   tests_dynamic();
   tests_typed();
 }