fix #26120, sideways casts no longer supported

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

Index: pkg/analyzer/lib/src/generated/type_system.dart
diff --git a/pkg/analyzer/lib/src/generated/type_system.dart b/pkg/analyzer/lib/src/generated/type_system.dart
index 6fcb63b250af71a920bee403b6a0c2075905a09f..c32e5c52b17a6949931542e34b0127b5702b816d 100644
--- a/pkg/analyzer/lib/src/generated/type_system.dart
+++ b/pkg/analyzer/lib/src/generated/type_system.dart
@@ -354,8 +354,7 @@ class StrongTypeSystemImpl extends TypeSystem {
 
     // Don't allow implicit downcasts between function types
     // and call method objects, as these will almost always fail.
-    if ((fromType is FunctionType && getCallMethodType(toType) != null) ||
-        (toType is FunctionType && getCallMethodType(fromType) != null)) {

[vsm, 2016/08/12 20:47:00]

Hmm, this feels odd to remove - doesn't this allow more casts now?

[Jennifer Messerly, 2016/08/12 21:00:28]

I was following instructions from TODO(vsm) here:
https://codereview.chromium.org/2231273002/diff/20001/pkg/analyzer/lib/src/ta...

[vsm, 2016/08/12 21:10:44]

I get it now.  We're guaranteed to return false later on if the red is true.  We
don't need to extra check here.

+    if (fromType is FunctionType && getCallMethodType(toType) != null) {
       return false;
     }
 
@@ -373,7 +372,7 @@ class StrongTypeSystemImpl extends TypeSystem {
 
     // If the subtype relation goes the other way, allow the implicit
     // downcast.
-    if (isSubtypeOf(toType, fromType) || toType.isAssignableTo(fromType)) {
+    if (isSubtypeOf(toType, fromType)) {
       // TODO(leafp,jmesserly): we emit warnings/hints for these in
       // src/task/strong/checker.dart, which is a bit inconsistent. That
       // code should be handled into places that use isAssignableTo, such as