fix #25578, implement @covariant parameter overrides

pkg/analyzer/lib/src/generated/error_verifier.dart

Index: pkg/analyzer/lib/src/generated/error_verifier.dart
diff --git a/pkg/analyzer/lib/src/generated/error_verifier.dart b/pkg/analyzer/lib/src/generated/error_verifier.dart
index 717ef2ad6defb872ebb81583d30b6426aa750be3..010833e30374c4f1edc65c8c0e87349d3c766386 100644
--- a/pkg/analyzer/lib/src/generated/error_verifier.dart
+++ b/pkg/analyzer/lib/src/generated/error_verifier.dart
@@ -5038,10 +5038,10 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
           FunctionType requiredMemberFT = _inheritanceManager
               .substituteTypeArgumentsInMemberFromInheritance(
                   requiredMemberType, memberName, enclosingType);
-          foundConcreteFT =
-              _typeSystem.typeToConcreteType(_typeProvider, foundConcreteFT);
-          requiredMemberFT =
-              _typeSystem.typeToConcreteType(_typeProvider, requiredMemberFT);
+          foundConcreteFT = _typeSystem.functionTypeToConcreteType(
+              _typeProvider, foundConcreteFT);
+          requiredMemberFT = _typeSystem.functionTypeToConcreteType(
+              _typeProvider, requiredMemberFT);
           if (_typeSystem.isSubtypeOf(foundConcreteFT, requiredMemberFT)) {
             continue;
           }
@@ -6229,9 +6229,9 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
   bool _expressionIsAssignableAtType(Expression expression,
       DartType actualStaticType, DartType expectedStaticType) {
     bool concrete = _options.strongMode && checker.isKnownFunction(expression);
-    if (concrete) {
-      actualStaticType =
-          _typeSystem.typeToConcreteType(_typeProvider, actualStaticType);
+    if (concrete && actualStaticType is FunctionType) {
+      actualStaticType = _typeSystem.functionTypeToConcreteType(
+          _typeProvider, actualStaticType);
       // TODO(leafp): Move the Downcast functionality here.
       // TODO(leafp): Support strict downcasts

[Leaf, 2016/09/12 23:42:37]

At least the second of these TODOs is dead, you've already done it... :)  

The point of the first TODO is lost in the mists of time, but I think it's also
dead.

[Jennifer Messerly, 2016/09/13 00:16:21]

Good catch, done!

     }