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

tests/html/indexeddb_1_test.dart

Index: tests/html/indexeddb_1_test.dart
diff --git a/tests/html/indexeddb_1_test.dart b/tests/html/indexeddb_1_test.dart
index d55494c2e00f792fc7168316df19b03fb7a17d60..8eda6c4316617988ff71759e766b20dbace6acf2 100644
--- a/tests/html/indexeddb_1_test.dart
+++ b/tests/html/indexeddb_1_test.dart
@@ -15,7 +15,7 @@ testReadWrite(key, value, matcher,
 
   fail(e) {
     guardAsync(() {
-      throw const Exception('IndexedDB failure');
+      throw new Exception('IndexedDB failure');
     });
   }
 
@@ -91,7 +91,7 @@ testReadWriteTyped(key, value, matcher,
 
   fail(e) {
     guardAsync(() {
-      throw const Exception('IndexedDB failure');
+      throw new Exception('IndexedDB failure');
     });
   }