Fix VM implementation of CastError not to extend TypeError (issue 5280).

tests/language/generic_test.dart

 // Copyright (c) 2011, 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.
 // VMOptions=--checked
 // Dart test program testing generic type allocations and generic type tests.
 import "package:expect/expect.dart";
 
 class A {
   const A();
 }
@@ skipping 38 matching lines @@
     Expect.equals(true, d.caa_.b_.isT(const AA()));
     C c = new C(const AA());  // c is of raw type C, T in C<T> is dynamic.
     Expect.equals(true, c.b_ is B);
     Expect.equals(true, c.b_ is B<AA>);
     Expect.equals(true, c.b_.isT(const AA()));
     Expect.equals(true, c.b_.isT(const AX()));
     try {
       E e = new E();  // Throws a type error, if type checks are enabled.
     } on TypeError catch (error) {
       result = 1;
-      int pos = error.url.lastIndexOf("/", error.url.length);
-      if (pos == -1) {
-        pos = error.url.lastIndexOf("\\", error.url.length);
-      }
-      String subs = error.url.substring(pos + 1, error.url.length);
-      Expect.equals("generic_test.dart", subs);
-      Expect.equals(31, error.line);  // new B<T>(t); AX does not extend A.
-      Expect.equals(21, error.column);
+      // Location of malformed error: T extends A, but AX does not extend A.
+      Expect.isTrue(error.toString().contains("line 20 pos 9"));
+      // Location of failed type check: new B<T>(t)/
+      Expect.isTrue(error.stackTrace.toString().contains(
+          "generic_test.dart:31:21"));
     }
     return result;
   }
 
   static testMain() {
     Expect.equals(1, test());
   }
 }
 
 
 main() {
   GenericTest.testMain();
 }