Fix check for throwing NSM implementations.

pkg/compiler/lib/src/js_backend/no_such_method_registry.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.
 
 part of js_backend;
 
 /**
  * Categorizes `noSuchMethod` implementations.
  *
  * If user code includes `noSuchMethod` implementations, type inference is
@@ skipping 153 matching lines @@
 
   bool _hasThrowingSyntax(FunctionElement element) {
     Statement body = element.node.body;
     if (body is Return && body.isArrowBody) {
       if (body.expression is Throw) {
         return true;
       }
     } else if (body is Block &&
         !body.statements.isEmpty &&
         body.statements.nodes.tail.isEmpty) {
-      if (body.statements.nodes.head is Throw) {
-        return true;
+      if (body.statements.nodes.head is ExpressionStatement) {
+        ExpressionStatement stmt = body.statements.nodes.head;
+        return stmt.expression is Throw;
       }
     }
     return false;
   }
 }
 
 enum NsmCategory { DEFAULT, THROWING, OTHER }