Fix null-termination bug in Stacktrace::ToCString.

tests/language/stacktrace_test.dart

+
+// Copyright (c) 2013, 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.
+
+// Test that a stack trace is properly terminated (issue 8850).
+void main() {
+  var ex = new Exception("fail");
+  try {
+    throw ex;
+  } on Exception catch(e, st) {
+    Expect.equals(ex, e);
+    Expect.isTrue(st.toString().endsWith("\n"));
+  }
+}