fixes #25640, errors on function downcasts when we know it's an exact type

pkg/analyzer/lib/src/task/strong/info.dart

Index: pkg/analyzer/lib/src/task/strong/info.dart
diff --git a/pkg/analyzer/lib/src/task/strong/info.dart b/pkg/analyzer/lib/src/task/strong/info.dart
index 4f39f6b347fcc41ae3c8a28fd177801ee58685df..d66847c6faaa7c4739a6de315c591f05fc8622a5 100644
--- a/pkg/analyzer/lib/src/task/strong/info.dart
+++ b/pkg/analyzer/lib/src/task/strong/info.dart
@@ -138,6 +138,18 @@ abstract class DownCast extends CoercionInfo {
       }
     }
 
+    Element staticElement = null;
+    if (expression is PropertyAccess) {
+      staticElement = expression.propertyName.staticElement;
+    } else if (expression is Identifier) {
+      staticElement = expression.staticElement;
+    }
+    // First class functions, where we know the original declaration, will have
+    // an exact type, so we know a downcast will fail.
+    if (staticElement is FunctionElement) {

[vsm, 2016/02/03 23:51:59]

Does this guarantee a static or top-level function (as opposed to a method)?

[Jennifer Messerly, 2016/02/04 00:41:30]

It includes top-level and local functions (named, lambdas). Does not include
methods (instance or static). Also does not include things that can store
first-class functions, like variables/parameters/fields.

The common base class between MethodElement, FunctionElement, and
PropertyAccessorElement is called ExecutableElement. (There is a further up
interface, called FunctionTypedElement, that includes typedefs as well.)

+      return new StaticTypeError(rules, expression, toT, reason: reason);
+    }
+
     // TODO(vsm): Change this to an assert when we have generic methods and
     // fix TypeRules._coerceTo to disallow implicit sideways casts.
     if (!rules.isSubtypeOf(toT, fromT)) {