Created a skeleton for Dart Debug Extension (#484)

Author: Pi Songkuntham

dwds/debug_extension/pubspec.yaml

@@ -0,0 +1,19 @@
+name: extension
+publish_to: none
+version: 1.0.0
+author: Dart Team <misc@dartlang.org>
+homepage: https://github.com/dart-lang/webdev
+description: >-
+  A chrome extension for Dart debugging.
+
+environment:
+  sdk: ">=2.3.2-dev.0.1 <3.0.0"
+
+dependencies:
+  js: ^0.6.1+1
+  sse: ^2.0.2
+
+dev_dependencies:
+  webdev: ^2.0.0
+  build_web_compilers: ">=1.2.0 <3.0.0"
+  build_runner: ">=1.5.0 <2.0.0"

dwds/debug_extension/web/background.dart

@@ -0,0 +1,112 @@
+// Copyright (c) 2019, 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.
+
+@JS('chrome')
+library background;
+
+import 'package:js/js.dart';
+import 'package:sse/client/sse_client.dart';
+
+// GENERATE:
+// pub run build_runner build web -o build -r
+void main() {
+  addListener(allowInterop((e) {
+    var query = QueryInfo(active: true, currentWindow: true);
+    Tab currentTab;
+
+    // Sends commands to debugger attached to the current tab.
+    //
+    // Extracts the extension backend port from the injected JS.
+    var callback = allowInterop((List<Tab> tabs) {
+      currentTab = tabs[0];
+      attach(Debuggee(tabId: currentTab.id), '1.3', allowInterop(() {}));
+      sendCommand(Debuggee(tabId: currentTab.id), 'Debugger.enable',
+          CommandParams(), allowInterop((e) {}));
+      sendCommand(Debuggee(tabId: currentTab.id), 'Runtime.evaluate',
+          CommandParams(expression: '\$extensionPort'),
+          allowInterop((RemoteObject e) {
+        var port = e.result.value;
+        startSseClient(port);
+      }));
+    });
+
+    queryTabs(query, allowInterop((List tabs) {
+      callback(List.from(tabs));
+    }));
+  }));
+}
+
+// Starts an SSE client.
+//
+// Creates 2 channels which connect to the SSE handler at the extension
+// backend, send a simple message.
+Future<void> startSseClient(port) async {
+  var channel = SseClient('http://localhost:$port/test');
+  channel.stream.listen((s) {
+    channel.sink.add('This is channel 1.');
+  }, onError: (e) {
+    channel.close();
+  }, cancelOnError: true);
+
+  var channel2 = SseClient('http://localhost:$port/test');
+  await channel2.onOpen.first;
+  channel2.sink.add('This is channel 2.');
+}
+
+@JS('browserAction.onClicked.addListener')
+external void addListener(Function callback);
+
+@JS('debugger.sendCommand')
+external void sendCommand(Debuggee target, String method,
+    CommandParams commandParams, Function callback);
+
+@JS('debugger.attach')
+external void attach(
+    Debuggee target, String requiredVersion, Function callback);
+
+@JS('tabs.query')
+external void queryTabs(QueryInfo queryInfo, Function callback);
+
+@JS()
+@anonymous
+class QueryInfo {
+  external bool get active;
+  external bool get currentWindow;
+  external factory QueryInfo({bool active, bool currentWindow});
+}
+
+@JS()
+@anonymous
+class Debuggee {
+  external dynamic get tabId;
+  external String get extensionId;
+  external String get targetId;
+  external factory Debuggee(
+      {dynamic tabId, String extensionId, String targetId});
+}
+
+@JS()
+@anonymous
+class CommandParams {
+  external String get expression;
+  external factory CommandParams({String expression});
+}
+
+@JS()
+@anonymous
+class Tab {
+  external int get id;
+}
+
+@JS()
+@anonymous
+class RemoteObject {
+  external EvaluationResult get result;
+}
+
+@JS()
+@anonymous
+class EvaluationResult {
+  external dynamic get value;
+}