Mark empty HashMap constructor as NoThrows.

tests/compiler/dart2js/unused_empty_map_test.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.
+
+// Ensure that unused empty HashMap nodes are dropped from the output.
+
+import 'package:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
+import 'memory_compiler.dart';
+
+const TEST_SOURCE = const {"main.dart": r"""
+void main() {
+  var x = {};
+  return;
+}
+"""};
+
+const HASHMAP_EMPTY_CONSTRUCTOR = r"LinkedHashMap_LinkedHashMap$_empty";
+
+main() {
+  var collector = new OutputCollector();
+  var compiler = compilerFor(TEST_SOURCE, outputProvider: collector);
+  asyncTest(() =>
+    compiler.run(Uri.parse('memory:main.dart')).then((_) {
+      String generated = collector.getOutput('', 'js');
+      Expect.isFalse(generated.contains(HASHMAP_EMPTY_CONSTRUCTOR));
+    })
+  );
+}