Add an option '--depfile' to produce ninja-style depfile for the gene…

…rated kernel. (flutter#4704)

* Add an option '--depfile' to produce ninja-style depfile for the generated kernel file.

* Use [program.uriToSource] as a source of dependencies.

frontend_server/lib/server.dart

@@ -57,6 +57,8 @@ ArgParser _argParser = new ArgParser(allowTrailingOptions: true)
   ..addOption('output-incremental-dill',
       help: 'Output path for the generated incremental dill',
       defaultsTo: null)
+  ..addOption('depfile',
+      help: 'Path to output Ninja depfile. Only used in batch mode.')
   ..addOption('packages',
       help: '.packages file to use for compilation',
       defaultsTo: null)
@@ -209,6 +211,12 @@ class _FrontendCompiler implements CompilerInterface {
       printer.writeProgramFile(program);
       await sink.close();
       _outputStream.writeln('$boundaryKey $_kernelBinaryFilename');
+
+      final String depfile = options['depfile'];
+      if (depfile != null) {
+        await _writeDepfile(program, _kernelBinaryFilename, depfile);
+      }
+
       _kernelBinaryFilename = _kernelBinaryFilenameIncremental;
     } else
       _outputStream.writeln(boundaryKey);
@@ -324,6 +332,23 @@ class _FrontendCompiler implements CompilerInterface {
   }
 }
 
+String _escapePath(String path) {
+  return path.replaceAll(r'\', r'\\').replaceAll(r' ', r'\ ');
+}
+
+// https://ninja-build.org/manual.html#_depfile
+Future<void> _writeDepfile(Program program, String output, String depfile) async {
+  final IOSink file = new File(depfile).openWrite();
+  file.write(_escapePath(output));
+  file.write(':');
+  for (Uri dep in program.uriToSource.keys) {
+    file.write(' ');
+    file.write(_escapePath(dep.toFilePath()));
+  }
+  file.write('\n');
+  await file.close();
+}
+
 /// Entry point for this module, that creates `_FrontendCompiler` instance and
 /// processes user input.
 /// `compiler` is an optional parameter so it can be replaced with mocked