initial bootstrap script for incremental compiler support (#43292)

* initial bootstrap script for incremental compiler support

* add more comments

* update to two scripts

packages/flutter_tools/lib/src/web/bootstrap.dart

@@ -0,0 +1,100 @@
+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:meta/meta.dart';
+
+/// The JavaScript bootstrap script to support in-browser hot restart.
+///
+/// The [requireUrl] loads our cached RequireJS script file. The [mapperUrl]
+/// loads the special Dart stack trace mapper. The [entrypoint] is the
+/// actual main.dart file.
+///
+/// This file is served when the browser requests "main.dart.js" in debug mode,
+/// and is responsible for bootstraping the RequireJS modules and attaching
+/// the hot reload hooks.
+String generateBootstrapScript({
+  @required String requireUrl,
+  @required String mapperUrl,
+  @required String entrypoint,
+}) {
+  return '''
+"use strict";
+
+// Attach source mapping.
+var mapperEl = document.createElement("script");
+mapperEl.defer = true;
+mapperEl.async = false;
+mapperEl.src = "$mapperUrl";
+document.head.appendChild(mapperEl);
+
+// Attach require JS.
+var requireEl = document.createElement("script");
+requireEl.defer = true;
+requireEl.async = false;
+requireEl.src = "$requireUrl";
+// This attribute tells require JS what to load as main (defined below).
+requireEl.setAttribute("data-main", "main_module");
+document.head.appendChild(requireEl);
+
+// Invoked by connected chrome debugger for hot reload/restart support.
+window.\$hotReloadHook = function(modules) {
+  return new Promise(function(resolve, reject) {
+    if (modules == null) {
+      reject();
+    }
+    // If no modules change, return immediately.
+    if (modules.length == 0) {
+      resolve();
+    }
+    var reloadCount = 0;
+    for (var i = 0; i < modules.length; i++) {
+      require.undef(modules[i]);
+      require([modules[i]], function(module) {
+        reloadCount += 1;
+        // once we've reloaded every module, trigger the hot reload.
+        if (reloadCount == modules.length) {
+          require(["$entrypoint", "dart_sdk"], function(app, dart_sdk) {
+            window.\$mainEntrypoint = app.main.main;
+            window.\$hotReload(resolve);
+          });
+        }
+      });
+    }
+  });
+}
+''';
+}
+
+/// Generate a synthetic main module which captures the application's main
+/// method.
+String generateMainModule({@required String entrypoint}) {
+  return '''
+// Create the main module loaded below.
+define("main_module", ["$entrypoint", "dart_sdk"], function(app, dart_sdk) {
+  dart_sdk.dart.setStartAsyncSynchronously(true);
+  dart_sdk._isolate_helper.startRootIsolate(() => {}, []);
+  dart_sdk._debugger.registerDevtoolsFormatter();
+  dart_sdk.ui.webOnlyInitializePlatform();
+
+  // Attach the main entrypoint and hot reload functionality to the window.
+  window.\$mainEntrypoint = app.main.main;
+  if (window.\$hotReload == null) {
+    window.\$hotReload = function(cb) {
+      dart_sdk.developer.invokeExtension("ext.flutter.disassemble", "{}");
+      dart_sdk.dart.hotRestart();
+      window.\$mainEntrypoint();
+      if (cb != null) {
+        cb();
+      }
+    }
+  }
+  app.main.main();
+});
+
+// Require JS configuration.
+require.config({
+  waitSeconds: 0,
+});
+''';
+}

packages/flutter_tools/test/general.shard/web/bootstrap_test.dart

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+// Copyright 2019 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_tools/src/web/bootstrap.dart';
+
+import '../../src/common.dart';
+
+void main() {
+  test('generateBootstrapScript embeds urls correctly', () {
+    final String result = generateBootstrapScript(
+      requireUrl: 'require.js',
+      mapperUrl: 'mapper.js',
+      entrypoint: 'foo/bar/main.js',
+    );
+    // require js source is interpolated correctly.
+    expect(result, contains('requireEl.src = "require.js";'));
+    // stack trace mapper source is interpolated correctly.
+    expect(result, contains('mapperEl.src = "mapper.js";'));
+    // data-main is set to correct bootstrap module.
+    expect(result, contains('requireEl.setAttribute("data-main", "main_module");'));
+    // bootstrap main module has correct imports.
+    expect(result, contains('require(["foo/bar/main.js", "dart_sdk"],'
+      ' function(app, dart_sdk) {'));
+  });
+
+  test('generateMainModule embeds urls correctly', () {
+    final String result = generateMainModule(
+      entrypoint: 'foo/bar/main.js',
+    );
+    // bootstrap main module has correct defined module.
+    expect(result, contains('define("main_module", ["foo/bar/main.js", "dart_sdk"], '
+      'function(app, dart_sdk) {'));
+  });
+}