Enforce single computeDelta() invocation.

pkg/front_end/test/incremental_kernel_generator_test.dart

Index: pkg/front_end/test/incremental_kernel_generator_test.dart
diff --git a/pkg/front_end/test/incremental_kernel_generator_test.dart b/pkg/front_end/test/incremental_kernel_generator_test.dart
index 8b4aaae5ad6b6f798fea6326c0d83859a63910aa..f61b356b06fbd9c5a470125291037b90bd23b7c4 100644
--- a/pkg/front_end/test/incremental_kernel_generator_test.dart
+++ b/pkg/front_end/test/incremental_kernel_generator_test.dart
@@ -60,7 +60,7 @@ class IncrementalKernelGeneratorTest {
   test_acceptLastDelta() async {
     writeFile('/test/.packages', 'test:lib/');
     String path = '/test/lib/test.dart';
-    Uri uri = writeFile(path, 'var v = 1;');
+    Uri uri = writeFile(path, '');
 
     await getInitialState(uri);
     incrementalKernelGenerator.acceptLastDelta();
@@ -223,6 +223,24 @@ static field asy::Future<core::String> b;
 ''');
   }
 
+  test_computeDelta_hasAnotherRunning() async {
+    writeFile('/test/.packages', 'test:lib/');
+    String path = '/test/lib/test.dart';
+    Uri uri = writeFile(path, '');
+
+    await getInitialState(uri);
+    incrementalKernelGenerator.acceptLastDelta();
+
+    // Run, but don't wait.
+    incrementalKernelGenerator.computeDelta();

[Siggi Cherem (dart-lang), 2017/08/10 22:43:29]

we might want to store the future returned by this method and await for it at
the end of this test, so we don't have a lingering work happening after the test
is done.

[scheglov, 2017/08/10 23:46:49]

Done.

+
+    // Run another, this causes StateError.
+    try {
+      await incrementalKernelGenerator.computeDelta();

[Siggi Cherem (dart-lang), 2017/08/10 22:43:29]

Could this be flaky?

In particular, is it at all possible for the first computeDelta to complete by
the time we reach this point? Or are we guaranteed that we have at least one
microtask pending (so it is not possible)? Right now it wouldn't be visible on
this test because if it completes early we would still throw because we haven't
`accepted` the delta, but if we address the comment below, this might be more
relevant.

[scheglov, 2017/08/10 23:46:49]

I changed computeDelta() to check _isComputeDeltaExecuting synchronously and set
it to false asynchronously, before returning the result.

+      fail('StateError expected.');
+    } on StateError {}

[Siggi Cherem (dart-lang), 2017/08/10 22:43:29]

there are two reasons why computeDelta could throw a state error, let's check
that the error you are getting is the one about computeDelta is executing

[scheglov, 2017/08/10 23:46:50]

Well, there is no great way to do this.
I had to use contain(string).

+  }
+
   test_inferPackagesFile() async {
     writeFile('/test/.packages', 'test:lib/');
     String aPath = '/test/lib/a.dart';