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

frog/leg/ssa/builder.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.
 
 class Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 668 matching lines @@
   void visitLogicalNot(Send node) {
     assert(node.argumentsNode is Prefix);
     visit(node.receiver);
     HNot not = new HNot(popBoolified());
     push(not);
   }
 
   void visitUnary(Send node, Operator op) {
     assert(node.argumentsNode is Prefix);
     visit(node.receiver);
+    if (op.source.stringValue == '+') return;
     HInstruction operand = pop();
     HInstruction target =
         new HStatic(interceptors.getPrefixOperatorInterceptor(op));
     add(target);
     switch (op.source.stringValue) {
       case "-": push(new HNegate(target, operand)); break;
       case "~": push(new HBitNot(target, operand)); break;
       default: unreachable();
     }
   }
@@ skipping 832 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     compiler.unimplemented('SsaBuilder.visitCatchBlock', node: node);
   }
 
   visitTypedef(Typedef node) {
     compiler.unimplemented('SsaBuilder.visitTypedef', node: node);
   }
 }