Reapply "Tweak public APIs and use them in patch_sdk, dart2js, and kernel-service.""

pkg/front_end/lib/src/fasta/testing/kernel_chain.dart

Index: pkg/front_end/lib/src/fasta/testing/kernel_chain.dart
diff --git a/pkg/front_end/lib/src/fasta/testing/kernel_chain.dart b/pkg/front_end/lib/src/fasta/testing/kernel_chain.dart
index 04ae0cfa6fa7f3eca9eaa6305050a6e8586666ed..bdb0ff71bacb40946129d9ee6bad1e44288a72db 100644
--- a/pkg/front_end/lib/src/fasta/testing/kernel_chain.dart
+++ b/pkg/front_end/lib/src/fasta/testing/kernel_chain.dart
@@ -28,10 +28,14 @@ import 'package:kernel/binary/ast_to_binary.dart' show BinaryPrinter;
 import 'package:kernel/binary/ast_from_binary.dart' show BinaryBuilder;
 
 import 'package:testing/testing.dart'
-    show ChainContext, Result, StdioProcess, Step;
+    show ChainContext, Result, StdioProcess, Step, TestDescription;
 
 import 'package:kernel/ast.dart' show Program;
 
+import 'package:front_end/front_end.dart';
+
+import 'patched_sdk_location.dart' show computePatchedSdk;
+
 class Print extends Step<Program, Program, ChainContext> {
   const Print();
 
@@ -170,6 +174,40 @@ class Copy extends Step<Program, Program, ChainContext> {
   }
 }
 
+/// A `package:testing` step that runs the `package:front_end` compiler to
+/// generate a kernel program for an individual file.
+///
+/// Most options are hard-coded, but if necessary they could be moved to the
+/// [CompileContext] object in the future.
+class Compile extends Step<TestDescription, Program, CompileContext> {
+  const Compile();
+
+  String get name => "fasta compilation";
+
+  Future<Result<Program>> run(
+      TestDescription description, CompileContext context) async {
+    Result<Program> result;
+    reportError(CompilationError error) {
+      result ??= fail(null, error.message);
+    }
+
+    Uri sdk = await computePatchedSdk();
+    Program p = await kernelForProgram(
+        description.uri,
+        new CompilerOptions()
+          ..sdkRoot = sdk
+          ..packagesFileUri = Uri.base.resolve('.packages')
+          ..strongMode = context.strongMode
+          ..linkedDependencies = [sdk.resolve('platform.dill')]
+          ..onError = reportError);
+    return result ??= pass(p);
+  }
+}
+
+abstract class CompileContext implements ChainContext {
+  bool get strongMode;
+}
+
 class BytesCollector implements Sink<List<int>> {
   final List<List<int>> lists = <List<int>>[];