Changed warning to hint for `x as T` where T is a method type variable.

pkg/compiler/lib/src/resolution/members.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.resolution.members;
 
 import '../common.dart';
 import '../common/names.dart' show Selectors;
 import '../common/resolution.dart' show Resolution;
 import '../compile_time_constants.dart';
@@ skipping 1124 matching lines @@
 
   /// Handle a type cast expression, like `a as T`.
   ResolutionResult handleAs(Send node) {
     Node expression = node.receiver;
     visitExpression(expression);
 
     Node typeNode = node.arguments.head;
     DartType type =
         resolveTypeAnnotation(typeNode, registerCheckedModeCheck: false);
 
-    // GENERIC_METHODS: Method type variables are not reified so we must warn
-    // about the error which will occur at runtime.
+    // GENERIC_METHODS: Method type variables are not reified, so we must inform
+    // the developer about the potentially bug-inducing semantics.
     if (type is MethodTypeVariableType) {
-      reporter.reportWarningMessage(
+      reporter.reportHintMessage(
           node, MessageKind.TYPE_VARIABLE_FROM_METHOD_CONSIDERED_DYNAMIC);
     }
 
     registry.registerTypeUse(new TypeUse.asCast(type));
     registry.registerSendStructure(node, new AsStructure(type));
     return const NoneResult();
   }
 
   /// Handle the unary expression of an unresolved unary operator [text], like
   /// the no longer supported `+a`.
@@ skipping 3584 matching lines @@
     }
     return const NoneResult();
   }
 }
 
 /// Looks up [name] in [scope] and unwraps the result.
 Element lookupInScope(
     DiagnosticReporter reporter, Node node, Scope scope, String name) {
   return Elements.unwrap(scope.lookup(name), reporter, node);
 }