Fix wrong canonicalization of signature class due to name collision (issue 6353).

tests/language/closure8_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.
+// Regression test for issue 6353.
+
+class A<E> {
+}
+
+class C<E> extends A<E> {
+  forEach(callback(E element)) {
+  }
+}
+
+class D<E> {
+  lala(E element) {
+  }
+}
+
+main() {
+  var c = new C<int>();
+  c.forEach(new D<int>().lala);
+}
+