Download secondary SDK (#29633)

packages/flutter_tools/lib/src/cache.dart

@@ -24,6 +24,7 @@ class Cache {
       _artifacts.add(MaterialFonts(this));
       _artifacts.add(FlutterEngine(this));
       _artifacts.add(GradleWrapper(this));
+      _artifacts.add(FlutterWebSdk(this));
     } else {
       _artifacts.addAll(artifacts);
     }
@@ -162,6 +163,12 @@ class Cache {
     return getCacheArtifacts().childDirectory(name);
   }
 
+  /// The web sdk has to be co-located with the dart-sdk so that they can share source
+  /// code.
+  Directory getWebSdkDirectory() {
+    return getRoot().childDirectory('flutter_web_sdk');
+  }
+
   String getVersionFor(String artifactName) {
     final File versionFile = fs.file(fs.path.join(_rootOverride?.path ?? flutterRoot, 'bin', 'internal', '$artifactName.version'));
     return versionFile.existsSync() ? versionFile.readAsStringSync().trim() : null;
@@ -357,6 +364,34 @@ class MaterialFonts extends CachedArtifact {
   }
 }
 
+/// A cached artifact containing the web dart:ui sources, platform dill files,
+/// and libraries.json.
+///
+/// This SDK references code within the regular Dart sdk to reduce download size.
+class FlutterWebSdk extends CachedArtifact {
+  FlutterWebSdk(Cache cache) : super('flutter_web_sdk', cache);
+
+  @override
+  Directory get location => cache.getWebSdkDirectory();
+
+  @override
+  String get version => cache.getVersionFor('engine');
+
+  @override
+  Future<void> updateInner() {
+    String platformName = 'flutter-web-sdk-';
+    if (platform.isMacOS) {
+      platformName += 'darwin-x64';
+    } else if (platform.isLinux) {
+      platformName += 'linux-x64';
+    } else if (platform.isWindows) {
+      platformName += 'windows-x64';
+    }
+    final Uri url = Uri.parse('$_storageBaseUrl/flutter_infra/flutter/$version/$platformName.zip');
+    return _downloadZipArchive('Downloading Web SDK...', url, location);
+  }
+}
+
 /// A cached artifact containing the Flutter engine binaries.
 class FlutterEngine extends CachedArtifact {
   FlutterEngine(Cache cache) : super('engine', cache);