Allow infering of return types on runtime helpers. It's only arugments the inferrer does not see.

tests/compiler/dart2js/no_such_method_codegen_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.
+
+// The mock compiler of dart2js used to make the compiler crash on
+// this program.
+//
+// The SSA backend generates a call to a throwNoSuchMethod helper for
+// the access to `foo`, and we used to not infer return types of
+// helpers, so we did not know throwNoSuchMethod was not returning.
+// As a consequence, all operator[] had to be compiled, and due to
+// missing backend dependencies, some of them were not resolved.
+
+import 'compiler_helper.dart';
+
+const String TEST = '''
+main() => foo[42];
+''';
+
+main() {
+  Uri uri = new Uri(scheme: 'source');
+  var compiler = compilerFor(TEST, uri);
+  compiler.runCompiler(uri);
+}