Use platform dill from the engine when compiling with dart2wasm. (#134970)

This is the final change needed to address https://github.com/flutter/flutter/issues/133467

This allows us to use the platform dill that is built by the engine when compiling apps. This also fixes the `--local-web-sdk` flag when compiling to wasm (which previously didn't work without some serious tweaking of the engine build output).

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

@@ -238,7 +238,8 @@ class Dart2WasmTarget extends Dart2WebTarget {
     );
     final File depFile = environment.buildDir.childFile('dart2wasm.d');
     final String dartSdkPath = artifacts.getArtifactPath(Artifact.engineDartSdkPath, platform: TargetPlatform.web_javascript);
-    final String dartSdkRoot = environment.fileSystem.directory(dartSdkPath).parent.path;
+    final String platformBinariesPath = getWebPlatformBinariesDirectory(artifacts, webRenderer).path;
+    final String platformFilePath = environment.fileSystem.path.join(platformBinariesPath, 'dart2wasm_platform.dill');
 
     final List<String> compilationArgs = <String>[
       artifacts.getArtifactPath(Artifact.engineDartAotRuntime, platform: TargetPlatform.web_javascript),
@@ -246,14 +247,7 @@ class Dart2WasmTarget extends Dart2WebTarget {
       artifacts.getArtifactPath(Artifact.dart2wasmSnapshot, platform: TargetPlatform.web_javascript),
       '--packages=.dart_tool/package_config.json',
       '--dart-sdk=$dartSdkPath',
-      '--multi-root-scheme',
-      'org-dartlang-sdk',
-      '--multi-root',
-      artifacts.getHostArtifact(HostArtifact.flutterWebSdk).path,
-      '--multi-root',
-      dartSdkRoot,
-      '--libraries-spec',
-      artifacts.getHostArtifact(HostArtifact.flutterWebLibrariesJson).path,
+      '--platform=$platformFilePath',
       if (buildMode == BuildMode.profile)
         '-Ddart.vm.profile=true'
       else

packages/flutter_tools/test/general.shard/build_system/targets/web_test.dart

@@ -36,14 +36,7 @@ const List<String> _kDart2WasmLinuxArgs = <String> [
   'Artifact.dart2wasmSnapshot.TargetPlatform.web_javascript',
   '--packages=.dart_tool/package_config.json',
   '--dart-sdk=Artifact.engineDartSdkPath.TargetPlatform.web_javascript',
-  '--multi-root-scheme',
-  'org-dartlang-sdk',
-  '--multi-root',
-  'HostArtifact.flutterWebSdk',
-  '--multi-root',
-  _kDartSdkRoot,
-  '--libraries-spec',
-  'HostArtifact.flutterWebLibrariesJson',
+  '--platform=HostArtifact.webPlatformKernelFolder/dart2wasm_platform.dill',
 ];
 
 const List<String> _kWasmOptLinuxArgrs = <String> [
@@ -58,10 +51,6 @@ const List<String> _kWasmOptLinuxArgrs = <String> [
   '--type-merging',
 ];
 
-/// The result of calling `.parent` on a Memory directory pointing to
-/// `'Artifact.engineDartSdkPath.TargetPlatform.web_javascript'`.
-const String _kDartSdkRoot = '.';
-
 void main() {
   late Testbed testbed;
   late Environment environment;