add check of preconditions in `run` step.

pkg/front_end/test/fasta/testing/suite.dart

Index: pkg/front_end/test/fasta/testing/suite.dart
diff --git a/pkg/front_end/test/fasta/testing/suite.dart b/pkg/front_end/test/fasta/testing/suite.dart
index d45a76ee2a2a827a892e7f75019548e7fa7aa505..1f9d70557287f3ea0e9cb74ac3891df39221426f 100644
--- a/pkg/front_end/test/fasta/testing/suite.dart
+++ b/pkg/front_end/test/fasta/testing/suite.dart
@@ -180,10 +180,12 @@ class Run extends Step<Uri, int, FastaContext> {
   bool get isRuntime => true;
 
   Future<Result<int>> run(Uri uri, FastaContext context) async {
+    if (context.platformUri == null) {
+      throw "Executed `Run` step before initializing the context.";
+    }
     File generated = new File.fromUri(uri);
     StdioProcess process;
     try {
-      await context.ensurePlatformUris();
       var platformDill = context.platformUri.toFilePath();
       var args = ['--platform=$platformDill', generated.path, "Hello, World!"];
       process = await StdioProcess.run(context.vm.toFilePath(), args);