Fix issue 14659: intrinsic method must check if the start, end arguments are Smi.

tests/language/substring_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.
+// Dart version of two-argument Ackermann-Peter function.
+
+import "package:expect/expect.dart";
+
+
+main() {
+  try {
+    print("abcdef".substring(1.5, 3.5));
+    Expect.fail("Should have thrown an exception");
+  } on TypeError catch (e) {
+    // OK.
+  } on ArgumentError catch (e) {
+    // OK.
+  }
+}