Remove cancel and make crash exit with code 253.

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 e1f4f33b141d9dd08bc6806d6af248e405ae8fa3..f50505a71f80b94f1987a0d4720a4abe098dd883 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -803,8 +803,8 @@ main() => new C();
       "in the implements clause.");
 
   static const MessageKind MULTI_INHERITANCE = const MessageKind(
-      "Inheritance of the same class with different type "
-      "arguments is not supported: Both #{firstType} and #{secondType} are "
+      "Dart2js does not currently support inheritance of the same class with "
+      "different type arguments: Both #{firstType} and #{secondType} are "
       "supertypes of #{thisType}.");
 
   static const MessageKind ILLEGAL_SUPER_SEND = const MessageKind(
@@ -1225,7 +1225,7 @@ main() => A.A = 1;
       );
 
   static const MessageKind UNSUPPORTED_LITERAL_SYMBOL = const MessageKind(
-      "Symbol literal '##{value}' is currently unsupported.");
+      "Symbol literal '##{value}' is currently unsupported by dart2js.");
 
   static const MessageKind INVALID_SYMBOL = const MessageKind('''
 '#{value}' is not a valid Symbol name because is not: