Generate a resolution error if a unary + is used for something other than numbers.

frog/leg/resolver.dart

 // Copyright (c) 2012, 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.
 
 interface TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
 }
 
 class TreeElementMapping implements TreeElements {
@@ skipping 605 matching lines @@
         NamedArgument named = argument;
         namedArguments.add(named.name.source);
       }
     }
     mapping.setSelector(node, new Invocation(count, namedArguments));
   }
 
   visitSend(Send node) {
     Element target = resolveSend(node);
     if (node.isOperator) {
-      if (node.selector.asIdentifier().source.stringValue === 'is') {
+      Operator op = node.selector.asOperator();
+      if (op.source.stringValue === 'is') {
         resolveTypeTest(node.arguments.head);
         assert(node.arguments.tail.isEmpty());
         mapping.setSelector(node, Selector.BINARY_OPERATOR);
       } else if (node.arguments.isEmpty()) {
+        if (node.isPrefix && op.source.stringValue === '+'
+            && node.receiver.asLiteralInt() === null
+            && node.receiver.asLiteralDouble() === null) {
+          error(node, MessageKind.EXPECTED_LITERAL_NUMBER);
+        }
         mapping.setSelector(node, Selector.UNARY_OPERATOR);
       } else {
         visit(node.argumentsNode);
         mapping.setSelector(node, Selector.BINARY_OPERATOR);
       }
     } else if (node.isIndex) {
       visit(node.argumentsNode);
       assert(node.arguments.tail.isEmpty());
       mapping.setSelector(node, Selector.INDEX);
     } else if (node.isPropertyAccess) {
@@ skipping 519 matching lines @@
 class TopScope extends Scope {
   LibraryElement library;
 
   TopScope(LibraryElement this.library) : super(null, null);
   Element lookup(SourceString name) => library.find(name);
 
   Element add(Element element) {
     throw "Cannot add an element in the top scope";
   }
 }