fix(dart/transform): Handle export cycles

Currently, an export cycle in dart inputs will cause the transformer to
hang indefinitely on the `DirectiveMetadataExtractor` step.

Closes #4370

modules_dart/transform/lib/src/transform/directive_metadata_extractor/extractor.dart

@@ -22,13 +22,16 @@ import 'package:code_transformers/assets.dart';
 /// [NgMeta] if there are no `Directive`-annotated classes in `entryPoint`.
 Future<NgMeta> extractDirectiveMetadata(
     AssetReader reader, AssetId entryPoint) {
-  return _extractDirectiveMetadataRecursive(reader, entryPoint);
+  return _extractDirectiveMetadataRecursive(
+      reader, entryPoint, new Set<AssetId>());
 }
 
-var _nullFuture = new Future.value(null);
+final _nullFuture = new Future.value(null);
 
 Future<NgMeta> _extractDirectiveMetadataRecursive(
-    AssetReader reader, AssetId entryPoint) async {
+    AssetReader reader, AssetId entryPoint, Set<AssetId> seen) async {
+  if (seen.contains(entryPoint)) return _nullFuture;
+  seen.add(entryPoint);
   var ngMeta = new NgMeta.empty();
   if (!(await reader.hasInput(entryPoint))) return ngMeta;
 
@@ -51,8 +54,12 @@ Future<NgMeta> _extractDirectiveMetadataRecursive(
     var assetId = uriToAssetId(entryPoint, uri, logger, null /* span */,
         errorOnAbsolute: false);
     if (assetId == entryPoint) return _nullFuture;
-    return _extractDirectiveMetadataRecursive(reader, assetId)
+    return _extractDirectiveMetadataRecursive(reader, assetId, seen)
-        .then(ngMeta.addAll);
+        .then((exportedNgMeta) {
+      if (exportedNgMeta != null) {
+        ngMeta.addAll(exportedNgMeta);
+      }
+    });
   }));
   return ngMeta;
 }

modules_dart/transform/test/transform/directive_metadata_extractor/all_tests.dart

@@ -193,6 +193,20 @@ void allTests() {
       expect(extracted.types['BazComponent'].selector).toEqual('[baz]');
     });
 
+    it('should handle `DirectiveMetadata` export cycles gracefully.', () async {
+      var extracted = await extractDirectiveMetadata(
+          reader,
+          new AssetId('a',
+              'directive_metadata_extractor/export_cycle_files/baz.ng_deps.dart'));
+      expect(extracted.types).toContain('FooComponent');
+      expect(extracted.types).toContain('BarComponent');
+      expect(extracted.types).toContain('BazComponent');
+
+      expect(extracted.types['FooComponent'].selector).toEqual('[foo]');
+      expect(extracted.types['BarComponent'].selector).toEqual('[bar]');
+      expect(extracted.types['BazComponent'].selector).toEqual('[baz]');
+    });
+
     it(
         'should include `DirectiveMetadata` from exported files '
         'expressed as absolute uris', () async {

modules_dart/transform/test/transform/directive_metadata_extractor/export_cycle_files/bar.ng_deps.dart

@@ -0,0 +1,19 @@
+library foo.ng_deps.dart;
+
+import 'bar.dart';
+import 'package:angular2/src/core/metadata.dart';
+
+export 'baz.dart';
+import 'baz.ng_deps.dart' as i0;
+
+var _visited = false;
+void initReflector(reflector) {
+  if (_visited) return;
+  _visited = true;
+  reflector
+    ..registerType(
+        BarComponent,
+        new ReflectionInfo(const [const Component(selector: '[bar]')], const [],
+            () => new BarComponent()));
+  i0.initReflector(reflector);
+}

modules_dart/transform/test/transform/directive_metadata_extractor/export_cycle_files/baz.ng_deps.dart

@@ -0,0 +1,17 @@
+library foo.ng_deps.dart;
+
+import 'baz.dart';
+import 'package:angular2/src/core/metadata.dart';
+
+export 'foo.dart';
+
+var _visited = false;
+void initReflector(reflector) {
+  if (_visited) return;
+  _visited = true;
+  reflector
+    ..registerType(
+        BazComponent,
+        new ReflectionInfo(const [const Component(selector: '[baz]')], const [],
+            () => new BazComponent()));
+}

modules_dart/transform/test/transform/directive_metadata_extractor/export_cycle_files/foo.ng_deps.dart

@@ -0,0 +1,19 @@
+library foo.ng_deps.dart;
+
+import 'foo.dart';
+import 'package:angular2/src/core/metadata.dart';
+
+export 'bar.dart';
+import 'bar.ng_deps.dart' as i0;
+
+var _visited = false;
+void initReflector(reflector) {
+  if (_visited) return;
+  _visited = true;
+  reflector
+    ..registerType(
+        FooComponent,
+        new ReflectionInfo(const [const Component(selector: '[foo]')], const [],
+            () => new FooComponent()));
+  i0.initReflector(reflector);
+}