[dartdevc] Fixing issue related to dual exports of multiple entrypoints in a cyclic import loop.

See: flutter/flutter#64011
Change-Id: I8a11e9d3f19428a516cf7a60ee45c3bacf26545d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/162383
Commit-Queue: Mark Zhou <[email protected]>
Reviewed-by: Nicholas Shahan <[email protected]>

Author: Markzipan

pkg/dev_compiler/lib/src/kernel/compiler.dart

@@ -867,16 +867,21 @@ class ProgramCompiler extends ComputeOnceConstantVisitor<js_ast.Expression>
     var savedTopLevelClass = _classEmittingExtends;
     _classEmittingExtends = c;
 
-    // Unroll mixins.
+    // Refers to 'S' in `class C extends S`. Set this to null to avoid
+    // referencing deferred supertypes in _emitClassConstructor's JS output.
+    js_ast.Expression baseClass;
+
     if (shouldDefer(supertype)) {
       deferredSupertypes.add(runtimeStatement('setBaseClass(#, #)', [
         getBaseClass(isMixinAliasClass(c) ? 0 : mixins.length),
         emitDeferredType(supertype),
       ]));
+      // Refers to 'supertype' without any type arguments.
       supertype =
           _coreTypes.rawType(supertype.classNode, _currentLibrary.nonNullable);
+    } else {
+      baseClass = emitClassRef(supertype);
     }
-    var baseClass = emitClassRef(supertype);
 
     if (isMixinAliasClass(c)) {
       // Given `class C = Object with M [implements I1, I2 ...];`

tests/language/export/dual_export_triangle_a.dart

@@ -0,0 +1,3 @@
+import 'dual_export_triangle_b.dart';
+
+class A extends B {}

tests/language/export/dual_export_triangle_b.dart

@@ -0,0 +1,3 @@
+import 'dual_export_triangle_c.dart';
+
+class B extends C {}

tests/language/export/dual_export_triangle_c.dart

@@ -0,0 +1,5 @@
+import 'dual_export_triangle_a.dart';
+
+class C0<T> {}
+
+class C extends C0<A> {}

tests/language/export/dual_export_triangle_entrypoint.dart

@@ -0,0 +1,3 @@
+import 'dual_export_triangle_a.dart';
+
+class Entrypoint {}

tests/language/export/dual_export_triangle_test.dart

@@ -0,0 +1,23 @@
+// Copyright (c) 2020, 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.
+
+// Test for cyclic export regression seen in: https://github.com/flutter/flutter/issues/64011
+
+// dual_export_triangle_entrypoint imports dual_export_triangle_a.
+// dual_export_triangle_a defines A (extends B).
+// dual_export_triangle_b defines B (extends C).
+// dual_export_triangle_c defines C (with type parameter A).
+// dual_export_triangle_test exports both dual_export_triangle_entrypoint and
+// either dual_export_triangle_b or dual_export_triangle_c.
+
+library dual_export_triangle_test;
+
+import 'dual_export_triangle_entrypoint.dart';
+
+export 'dual_export_triangle_entrypoint.dart';
+export 'dual_export_triangle_b.dart';
+
+main() {
+  print(Entrypoint());
+}