Return generated kernel filename on incremental compilation results. (#4069)

frontend_server/lib/server.dart

@@ -158,7 +158,7 @@ class _FrontendCompiler implements CompilerInterface {
     final IOSink sink = new File(_kernelBinaryFilename).openWrite();
     final BinaryPrinter printer = printerFactory.newBinaryPrinter(sink);
     printer.writeProgramFile(deltaProgram.newProgram);
-    _outputStream.writeln("$boundaryKey");
+    _outputStream.writeln("$boundaryKey $_kernelBinaryFilename");
     await sink.close();
     return null;
   }

frontend_server/test/server_test.dart

@@ -222,7 +222,7 @@ Future<int> main() async {
               boundaryKey = s.substring(RESULT_OUTPUT_SPACE.length);
             }
           } else {
-            if (s == boundaryKey) {
+            if (s.startsWith(boundaryKey)) {
               boundaryKey = null;
               receivedResult.sendPort.send(true);
             }