add check of preconditions in `run` step.

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

 // Copyright (c) 2016, the Dart project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
 library fasta.testing.suite;
 
 import 'dart:async' show Future;
 
 import 'dart:io' show File;
 
@@ skipping 162 matching lines @@
 class Run extends Step<Uri, int, FastaContext> {
   const Run();
 
   String get name => "run";
 
   bool get isAsync => true;
 
   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);
       print(process.output);
     } finally {
       generated.parent.delete(recursive: true);
     }
     return process.toResult();
   }
 }
@@ skipping 59 matching lines @@
             return fail(null, instrumentation.problemsAsString);
           }
         }
       }
     } on InputError catch (e, s) {
       return fail(null, e.error, s);
     }
     return pass(p);
   }
 }