Relaxes treatment of method type parameter in `as` expressions.

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index e17157442c72e8251650e482d8a175d88d0797aa..d93ef4dad3b003b0a7a0b03bc46d0ea574aae8eb 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -431,6 +431,7 @@ enum MessageKind {
   TYPE_VARIABLE_IN_CONSTANT,
   TYPE_VARIABLE_WITHIN_STATIC_MEMBER,
   TYPE_VARIABLE_FROM_METHOD_NOT_REIFIED,
+  TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC,
   TYPEDEF_FORMAL_WITH_DEFAULT,
   UNARY_OPERATOR_BAD_ARITY,
   UNBOUND_LABEL,
@@ -1233,7 +1234,17 @@ main() => f<int>();
 bool f<T>(Object o) => o is T;
 
 main() => f<int>(42);
-""",
+"""
+          ]),
+
+      MessageKind.TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC:
+      const MessageTemplate(
+          MessageKind.TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC,
+          "Method type variables are treated as `dynamic` in `as` expressions.",
+          options: const ["--generic-method-syntax"],
+          howToFix: "Try using the upper bound of the type variable, or check "
+              "that the blind success of the test does not introduce bugs.",
+          examples: const [
             """
 // Method type variables are not reified, so they cannot be tested dynamically.
 bool f<T>(Object o) => o as T;