Add unary arithmetic (+#, -#) to ThrowBehaviorVisitor

pkg/compiler/lib/src/native/js.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.
 
 part of native;
 
 class SideEffectsVisitor extends js.BaseVisitor {
   final SideEffects sideEffects;
   SideEffectsVisitor(this.sideEffects);
 
@@ skipping 205 matching lines @@
 
   NativeThrowBehavior visitThrow(js.Throw node) {
     return NativeThrowBehavior.MUST;
   }
 
   NativeThrowBehavior visitPrefix(js.Prefix node) {
     if (node.op == 'typeof' && node.argument is js.VariableUse)
       return NativeThrowBehavior.NEVER;
     NativeThrowBehavior result = visit(node.argument);
     switch (node.op) {
+      case '+':
+      case '-':
       case '!':
       case '~':
       case 'void':
       case 'typeof':
         return result;
       default:
         return NativeThrowBehavior.MAY;
     }
   }
 
@@ skipping 22 matching lines @@
         receiver.isPositional &&
         receiver.nameOrPosition == 0) {
       first = NativeThrowBehavior.MAY_THROW_ONLY_ON_FIRST_ARGUMENT_ACCESS;
     } else {
       first = NativeThrowBehavior.MAY;
     }
 
     return sequence(first, second);
   }
 }