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 49ab8d3481d5aabc99f084861a360536b10d6b32..dcbedbc151ec0dedc9410e32c24c6b3a10efc379 100644
--- a/pkg/front_end/test/fasta/testing/suite.dart
+++ b/pkg/front_end/test/fasta/testing/suite.dart
@@ -176,10 +176,13 @@ class Run extends Step<Uri, int, FastaContext> {
   bool get isRuntime => true;
 
   Future<Result<int>> run(Uri uri, FastaContext context) async {
+    if (context.platformUri == null) {
+      return new Result<int>.fail(
+          1, "Executed `Run` step before initializing the context.");

[Siggi Cherem (dart-lang), 2017/05/31 21:15:43]

not sure if you prefer to indicate these checks as failures or simply assert or
throw and let the testing report it as a crash.

Happy to change it however you like.

[ahe, 2017/06/01 10:45:43]

Sounds like a crash to me.

+    }
     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);