[flutter_tools] supports tree-shake-icons for web builds (#51808)

* [flutter_tools] supports tree-shake-icons for web builds

* Update compile.dart

* fix icon_tree_shaker

packages/flutter_tools/lib/src/build_system/targets/icon_tree_shaker.dart

@@ -289,7 +289,7 @@ class IconTreeShaker {
     for (final Map<String, dynamic> iconDataMap in consts.constantInstances) {
       if ((iconDataMap['fontPackage'] ?? '') is! String || // Null is ok here.
            iconDataMap['fontFamily'] is! String ||
-           iconDataMap['codePoint'] is! int) {
+           iconDataMap['codePoint'] is! num) {
         throw IconTreeShakerException._(
           'Invalid ConstFinder result. Expected "fontPackage" to be a String, '
           '"fontFamily" to be a String, and "codePoint" to be an int, '
@@ -301,7 +301,7 @@ class IconTreeShaker {
         ? family
         : 'packages/$package/$family';
       result[key] ??= <int>[];
-      result[key].add(iconDataMap['codePoint'] as int);
+      result[key].add((iconDataMap['codePoint'] as num).round());
     }
     return result;
   }

packages/flutter_tools/lib/src/build_system/targets/web.dart

@@ -156,36 +156,51 @@ class Dart2JSTarget extends Target {
     final BuildMode buildMode = getBuildModeForName(environment.defines[kBuildMode]);
     final String specPath = globals.fs.path.join(globals.artifacts.getArtifactPath(Artifact.flutterWebSdk), 'libraries.json');
     final String packageFile = PackageMap.globalPackagesPath;
+    final File outputKernel = environment.buildDir.childFile('app.dill');
     final File outputFile = environment.buildDir.childFile('main.dart.js');
 
-    final ProcessResult result = await globals.processManager.run(<String>[
+    // Run the dart2js compilation in two stages, so that icon tree shaking can
+    // parse the kernel file for web builds.
+    final ProcessResult kernelResult = await globals.processManager.run(<String>[
       globals.artifacts.getArtifactPath(Artifact.engineDartBinary),
       globals.artifacts.getArtifactPath(Artifact.dart2jsSnapshot),
       '--libraries-spec=$specPath',
-      if (dart2jsOptimization != null)
-        '-$dart2jsOptimization'
-      else
-        '-O4',
-      if (buildMode == BuildMode.profile)
-        '--no-minify',
       '-o',
-      outputFile.path,
+      outputKernel.path,
       '--packages=$packageFile',
       if (buildMode == BuildMode.profile)
         '-Ddart.vm.profile=true'
       else
         '-Ddart.vm.product=true',
-      if (csp)
-        '--csp',
       for (final String dartDefine in parseDartDefines(environment))
         '-D$dartDefine',
+      '--cfe-only',
       environment.buildDir.childFile('main.dart').path,
     ]);
-    if (result.exitCode != 0) {
-      throw Exception(result.stdout + result.stderr);
+    if (kernelResult.exitCode != 0) {
+      throw Exception(kernelResult.stdout + kernelResult.stderr);
+    }
+    final ProcessResult javaScriptResult = await globals.processManager.run(<String>[
+      globals.artifacts.getArtifactPath(Artifact.engineDartBinary),
+      globals.artifacts.getArtifactPath(Artifact.dart2jsSnapshot),
+      '--libraries-spec=$specPath',
+      if (dart2jsOptimization != null)
+        '-$dart2jsOptimization'
+      else
+        '-O4',
+      if (buildMode == BuildMode.profile)
+        '--no-minify',
+      if (csp)
+        '--csp',
+      '-o',
+      outputFile.path,
+      environment.buildDir.childFile('app.dill').path,
+    ]);
+    if (javaScriptResult.exitCode != 0) {
+      throw Exception(javaScriptResult.stdout + javaScriptResult.stderr);
     }
     final File dart2jsDeps = environment.buildDir
-      .childFile('main.dart.js.deps');
+      .childFile('app.dill.deps');
     if (!dart2jsDeps.existsSync()) {
       globals.printError('Warning: dart2js did not produced expected deps list at '
         '${dart2jsDeps.path}');
@@ -197,7 +212,7 @@ class Dart2JSTarget extends Target {
       platform: globals.platform,
     );
     final Depfile depfile = depfileService.parseDart2js(
-      environment.buildDir.childFile('main.dart.js.deps'),
+      environment.buildDir.childFile('app.dill.deps'),
       outputFile,
     );
     depfileService.writeToFile(

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

@@ -11,6 +11,7 @@ import '../base/logger.dart';
 import '../build_info.dart';
 import '../build_system/build_system.dart';
 import '../build_system/targets/dart.dart';
+import '../build_system/targets/icon_tree_shaker.dart';
 import '../build_system/targets/web.dart';
 import '../convert.dart';
 import '../globals.dart' as globals;
@@ -52,8 +53,7 @@ Future<void> buildWeb(
         kHasWebPlugins: hasWebPlugins.toString(),
         kDartDefines: jsonEncode(dartDefines),
         kCspMode: csp.toString(),
-        // TODO(dnfield): Enable font subset. We need to get a kernel file to do
-        // that. https://github.com/flutter/flutter/issues/49730
+        kIconTreeShakerFlag: buildInfo.treeShakeIcons.toString(),
       },
     ));
     if (!result.success) {