Support transforming the exception in Future.transform().

lib/coreimpl/future_implementation.dart

 // Copyright 2012 Google Inc. All Rights Reserved.
 // Dart core library.
 
 class FutureImpl<T> implements Future<T> {
 
   bool _isComplete = false;
 
   /**
    * Value that was provided to this Future by the Completer
    */
@@ skipping 151 matching lines @@
       throw new IllegalArgumentException(null);
     }
     if (_isComplete) {
       throw new FutureAlreadyCompleteException();
     }
     _exception = exception;
     _stackTrace = stackTrace;
     _complete();
   }
 
-  Future transform(Function transformation) {
+  Future transform(Function transformation, [onException(Object exception)]) {
     final completer = new Completer();
-    handleException((e) {
-      completer.completeException(e, this.stackTrace);
+
+    transformException(ex, stackTrace) {
+      var result = false;
+      if (onException != null) {
+        try {
+          result = onException(ex);
+        } catch (innerException) {
+          ex = innerException;
+        }
+      }
+
+      if (result != true) {
+        completer.completeException(ex, stackTrace);
+      }
+    }
+
+    handleException((ex) {
+      transformException(ex, this.stackTrace);
       return true;
     });
+
     then((v) {
       var transformed = null;
       try {
         transformed = transformation(v);
       } catch (ex, stackTrace) {
-        completer.completeException(ex, stackTrace);
+        transformException(ex, stackTrace);
         return;
       }
       completer.complete(transformed);
     });
+
     return completer.future;
   }
 
   Future chain(Function transformation) {
     final completer = new Completer();
     handleException((e) {
       completer.completeException(e, this.stackTrace);
       return true;
     });
     then((v) {
@@ skipping 25 matching lines @@
   }
 
   void complete(T value) {
     _futureImpl._setValue(value);
   }
 
   void completeException(Object exception, [Object stackTrace]) {
     _futureImpl._setException(exception, stackTrace);
   }
 }