cps-ir: Add support for intercepted calls.

tests/compiler/dart2js/js_backend_cps_ir_interceptors.dart

+// Copyright (c) 2014, 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.
+
+// Tests of interceptors.
+
+library interceptors_tests;
+
+import 'js_backend_cps_ir_test.dart';
+
+const List<TestEntry> tests = const [
+  const TestEntry("""
+main() {
+  var g = 1;
+
+  var x = g + 3;
+  print(x);
+}""",
+r"""
+function() {
+  var g, v0;
+  g = 1;
+  v0 = 3;
+  P.print(J.getInterceptor(g).$add(g, v0));
+  return null;
+}"""),
+];