Check const expressions for malformed types.

sdk/lib/_internal/compiler/implementation/warnings.dart

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index 45803c190a41f09ecac762152cd38093f47a27ac..fb33d49c1e11ecdec11f809c0cd70d52441fd712 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -527,7 +527,20 @@ main() => new C(0);"""]);
           "within a static member."));
 
   static const MessageKind TYPE_VARIABLE_IN_CONSTANT = const MessageKind(
-      "Error: Cannot refer to type variable in constant.");
+      "Error: Constant expressions can't refer to type variables.",
+      howToFix: "Try removing the type variable or replacing it with a "
+                "concrete type.",
+      examples: const ["""
+class C<T> {
+  const C();
+
+  m(T t) => const C<T>();
+}
+
+void main() => new C().m(null);  
+"""
+]);
+
 
   static const MessageKind INVALID_TYPE_VARIABLE_BOUND = const MessageKind(
       "Warning: '#{typeArgument}' is not a subtype of bound '#{bound}' for "