Skip directly to the last library to set the entry point and check importUri

pkg/front_end/lib/src/incremental_kernel_generator_impl.dart

 // Copyright (c) 2017, 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 file.
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:front_end/file_system.dart';
 import 'package:front_end/incremental_kernel_generator.dart';
 import 'package:front_end/src/base/api_signature.dart';
@@ skipping 118 matching lines @@
       }
 
       if (watch != null) _fsState.fileUris.forEach((f) => watch(f, true));
 
       // TODO(scheglov) Add libraries which import changed libraries.
       // For now the corresponding test works because we use full library
       // contents to compute signatures (not just API parts). So, every library
       // that imports a changed one, is affected.
 
       // Set the main method.
-      for (var library in program.libraries) {
-        if (library.fileUri == _entryPoint.toString()) {
-          program.mainMethod = library.procedures.firstWhere(
-              (procedure) => procedure.name.name == 'main',
-              orElse: () => null);
-          break;
-        }
+      if (program.libraries.isNotEmpty) {
+        var entryLibrary = program.libraries.last;

[scheglov, 2017/06/07 21:31:16]

I thought about doing this, but decided that this is not perfectly safe. What if
the last strongly connected component (with the entry point) consists of two
libraries, and it happens that the entry point library is not the last one in
this component? Topological sorting does not guarantee this to us.

[Siggi Cherem (dart-lang), 2017/06/07 21:36:10]

Good point, what do you think of either of these two ideas:
 - go backwards with program.libraries.reversed (it's likely closer to the end)
 - only visit libraries in results.last.kernelLibraries?

[scheglov, 2017/06/07 21:38:42]

Yeah, we know for sure that it is in the last result.

+        assert(entryLibrary.importUri == _entryPoint);
+        program.mainMethod = entryLibrary.procedures.firstWhere(
+            (procedure) => procedure.name.name == 'main',
+            orElse: () => null);
       }
 
       return new DeltaProgram(program);
     });
   }
 
   @override
   void invalidate(Uri uri) {
     _invalidatedFiles.add(uri);
   }
@@ skipping 169 matching lines @@
   /// TODO(scheglov) Use API signatures.
   /// TODO(scheglov) Or use tree shaking and compute signatures of outlines.
   final String signature;
 
   /// Kernel libraries for libraries in the [cycle].  Bodies of dependencies
   /// are not included, but but references to those dependencies are included.
   final List<Library> kernelLibraries;
 
   _LibraryCycleResult(this.cycle, this.signature, this.kernelLibraries);
 }