Silence dart2js's success output.

lib/src/runner/browser/compiler_pool.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:async/async.dart';
 import 'package:path/path.dart' as p;
 import 'package:pool/pool.dart';
 
 import '../../util/io.dart';
 import '../configuration.dart';
 import '../load_exception.dart';
 
+/// A regular expression matching the first status line printed by dart2js.
+final _dart2jsStatus =
+    new RegExp(r"^Dart file \(.*\) compiled to JavaScript: .*\n?");
+
 /// A pool of `dart2js` instances.
 ///
 /// This limits the number of compiler instances running concurrently.
 class CompilerPool {
   /// The internal pool that controls the number of process running at once.
   final Pool _pool;
 
   /// The test runner configuration.
   final Configuration _config;
 
@@ skipping 68 matching lines @@
 
         await Future.wait([
           _printOutputStream(process.stdout, buffer),
           _printOutputStream(process.stderr, buffer),
         ]);
 
         var exitCode = await process.exitCode;
         _processes.remove(process);
         if (_closed) return;
 
-        if (buffer.isNotEmpty) print(buffer);
+        var output = buffer.toString().replaceFirst(_dart2jsStatus, '');
+        if (output.isNotEmpty) print(output);
 
         if (exitCode != 0) throw new LoadException(dartPath, "dart2js failed.");
 
         _fixSourceMap(jsPath + '.map');
       });
     });
   }
 
   // TODO(nweiz): Remove this when sdk#17544 is fixed.
   /// Fix up the source map at [mapPath] so that it points to absolute file:
@@ skipping 26 matching lines @@
   /// have been killed and all resources released.
   Future close() {
     return _closeMemo.runOnce(() async {
       await Future.wait(_processes.map((process) async {
         process.kill();
         await process.exitCode;
       }));
     });
   }
 }