Bug fix - use type of input, not type of result, for throw-on-null check

tests/compiler/dart2js/js_spec_optimization_test.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'compiler_helper.dart';
 
 const String TEST_1 = r"""
@@ skipping 39 matching lines @@
         '#.toLowerCase()', s);
     var s2 = JS('returns:String;depends:none;effects:none;throws:null(1)',
         '#.toUpperCase()', s);
     print(s2);
 
     // absent: 'toLowerCase' - removed since s.toUpperCase() generates the same
     // noSuchMethod.
   }
 """;
 
+const String TEST_4 = r"""
+  import 'dart:_foreign_helper';
+  main() {
+    var s = JS('String|Null', '"Hello"');
+    var s1 = JS('returns:String;depends:none;effects:none;throws:null(1)',
+        '#.toLowerCase()', s);
+    var s2 = JS('returns:String;depends:none;effects:none;throws:null(1)',
+        '#.toUpperCase()', s);
+
+    // present: 'erCase' - retained at least one call to guarantee exception.
+  }
+""";
+
+const String TEST_5 = r"""
+  import 'dart:_foreign_helper';
+  main() {
+    var s = JS('String', '"Hello"');
+    var s1 = JS('returns:String;depends:none;effects:none;throws:null(1)',
+        '#.toLowerCase()', s);
+    var s2 = JS('returns:String;depends:none;effects:none;throws:null(1)',
+        '#.toUpperCase()', s);
+
+    // absent: 'erCase' - neither call needs to be retained since there is no
+    // exception.
+  }
+""";
+
 main() {
   RegExp directivePattern = new RegExp(
       //      \1                    \2        \3
       r'''// *(present|absent): (?:"([^"]*)"|'([^'']*)')''',
       multiLine: true);
 
   Future check(String test) {
     Uri uri = new Uri(scheme: 'dart', path: 'test');
     var compiler = compilerFor(test, uri, expectedErrors: 0);
     return compiler.run(uri).then((_) {
@@ skipping 13 matching lines @@
               "Must not find '$pattern' in:\n$generated");
         }
       }
     });
   }
 
   asyncTest(() => Future.wait([
         check(TEST_1),
         check(TEST_2),
         check(TEST_3),
+        check(TEST_4),
+        check(TEST_5),
       ]));
 }