Fix unbound type reference in extension method problem (#2741)

Author: jcollins-g

lib/src/model/type_parameter.dart

@@ -69,9 +69,13 @@ class TypeParameter extends ModelElement {
 
   @override
   Map<String, CommentReferable> get referenceChildren {
-    return _referenceChildren ??= {
-      boundType.name: boundType,
-    };
+    if (_referenceChildren == null) {
+      _referenceChildren = {};
+      if (boundType != null) {
+        _referenceChildren[boundType.name] = boundType;
+      }
+    }
+    return _referenceChildren;
   }
 
   @override

test/end2end/model_test.dart

@@ -2257,13 +2257,19 @@ void main() {
       Class TypeParameterThings,
           TypeParameterThingsExtended,
           TypeParameterThingsExtendedQ;
-      Field aName, aThing;
+      Extension UnboundTypeTargetExtension;
+      Field aName, aThing, doesNotCrash;
       TypeParameter ATypeParam, BTypeParam, CTypeParam, DTypeParam, QTypeParam;
       Method aMethod, aMethodExtended, aMethodExtendedQ;
       Parameter aParam, anotherParam, typedParam;
       ModelFunction aTopLevelTypeParameterFunction;
 
       setUpAll(() {
+        UnboundTypeTargetExtension = fakeLibrary.extensions
+            .firstWhere((f) => f.name == 'UnboundTypeTargetExtension');
+        doesNotCrash = UnboundTypeTargetExtension.instanceFields
+            .firstWhere((f) => f.name == 'doesNotCrash');
+
         aTopLevelTypeParameterFunction = fakeLibrary.functions
             .firstWhere((f) => f.name == 'aTopLevelTypeParameterFunction');
         // TODO(jcollins-g): dart-lang/dartdoc#2704, HTML and type parameters
@@ -2306,6 +2312,11 @@ void main() {
             .firstWhere((p) => p.name == 'QTypeParam');
       });
 
+      test('on extension targeting an unbound type', () {
+        expect(newLookup(UnboundTypeTargetExtension, 'doesNotCrash'),
+            equals(MatchingLinkResult(doesNotCrash)));
+      });
+
       test('on inherited documentation', () {
         expect(newLookup(aMethodExtended, 'ATypeParam'),
             equals(MatchingLinkResult(ATypeParam)));

testing/test_package/lib/fake.dart

@@ -1087,6 +1087,12 @@ extension Leg on Megatron<String> {
   bool get hasRightLeg => true;
 }
 
+/// Refer to [doesNotCrash] here.
+extension UnboundTypeTargetExtension<T> on T {
+  /// We hope so!
+  bool get doesNotCrash => true;
+}
+
 class Megatron<T> {}
 
 class SuperMegaTron<T extends String> extends Megatron<String> {}