Change and structure how Stream implementations do callbacks.

sdk/lib/async/async_error.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.async;
 
 /**
  * Error result of an asynchronous computation.
  */
 class AsyncError {
   /** The actual error thrown by the computation. */
   final error;
   /** Stack trace corresponding to the error, if available. */
   final Object stackTrace;
   /** Asynchronous error leading to this error, if error handling fails. */
   final AsyncError cause;
 
   // TODO(lrn): When possible, combine into one constructor with both optional
   // positional and named arguments.
   AsyncError(this.error, [this.stackTrace]): cause = null;
   AsyncError.withCause(this.error, this.stackTrace, this.cause);
 
-  void _writeOn(StringBuffer buffer) {
-    buffer.add("'");
+  void _writeOn(StringSink buffer) {
+    buffer.write("'");
     String message;
     try {
       message = error.toString();
     } catch (e) {
       message = Error.safeToString(error);
     }
-    buffer.add(message);
-    buffer.add("'\n");
+    buffer.write(message);
+    buffer.write("'\n");
     if (stackTrace != null) {
-      buffer.add("Stack trace:\n");
-      buffer.add(stackTrace.toString());
-      buffer.add("\n");
+      buffer.write("Stack trace:\n");
+      buffer.writeln(stackTrace.toString());
     }
   }
 
   String toString() {
     StringBuffer buffer = new StringBuffer();
-    buffer.add("AsyncError: ");
+    buffer.write("AsyncError: ");
     _writeOn(buffer);
     AsyncError cause = this.cause;
     while (cause != null) {
-      buffer.add("Caused by: ");
+      buffer.write("Caused by: ");
       cause._writeOn(buffer);
       cause = cause.cause;
     }
     return buffer.toString();
   }
 
   throwDelayed() {
     reportError() {
       print("Uncaught Error: $error");
       if (stackTrace != null) {
         print("Stack Trace:\n$stackTrace\n");
       }
     }
 
     try {
       Timer.run(() {
         reportError();
         // TODO(floitsch): we potentially want to call the global error handler
         // directly so that we can pass the stack trace.
         throw error;
       });
     } catch (e) {
       // Unfortunately there is not much more we can do...
       reportError();
     }
   }
 }