Skip source files attributed to 'dart:' libraries when writing a dep file.

Change-Id: I656edd730c1bb2433768646295cf3ab4325819c1
Reviewed-on: https://dart-review.googlesource.com/c/77985
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

pkg/vm/lib/frontend_server.dart

@@ -636,12 +636,27 @@ String _escapePath(String path) {
   return path.replaceAll(r'\', r'\\').replaceAll(r' ', r'\ ');
 }
 
+List<Uri> _getDependencies(Component component) {
+  var deps = <Uri>[];
+  for (Library lib in component.libraries) {
+    if (lib.importUri.scheme == 'dart') {
+      continue;
+    }
+    deps.add(lib.fileUri);
+    for (LibraryPart part in lib.parts) {
+      final Uri fileUri = lib.fileUri.resolve(part.partUri);
+      deps.add(fileUri);
+    }
+  }
+  return deps;
+}
+
 // https://ninja-build.org/manual.html#_depfile
 _writeDepfile(Component component, String output, String depfile) async {
   final IOSink file = new File(depfile).openWrite();
   file.write(_escapePath(output));
   file.write(':');
-  for (Uri dep in component.uriToSource.keys) {
+  for (Uri dep in _getDependencies(component)) {
     if (dep == null) continue;
     file.write(' ');
     file.write(_escapePath(dep.toFilePath()));