Log when dart2js starts compiling.

sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_until_its_output_is_requested_test.dart

Index: sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_until_its_output_is_requested_test.dart
diff --git a/sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_until_its_output_is_requested_test.dart b/sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_until_its_output_is_requested_test.dart
index e71c925b9ea1c7cf6004166499bed85bb9bd4cc1..f26c60f6b8f8fa025b23d980c2603b40f2cffba4 100644
--- a/sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_until_its_output_is_requested_test.dart
+++ b/sdk/lib/_internal/pub/test/transformer/dart2js/does_not_compile_until_its_output_is_requested_test.dart
@@ -32,6 +32,10 @@ main() {
     // Once we request the output, it should start compiling and fail.
     requestShould404("syntax-error.dart.js");
     expect(server.nextLine(),
+        completion(equals("[Info from Dart2JS]:")));
+    expect(server.nextLine(),
+        completion(equals("Compiling myapp|web/syntax-error.dart...")));
+    expect(server.nextLine(),
         completion(equals("Build completed with 1 errors.")));
     endPubServe();
   });