repl: support standalone blocks

Enable support for standalone block statements.
```js
node 🙈 ₹ git:(upstream ⚡ bare-block) ./node
> { var x = 3; console.log(x); }
3
undefined
> {}
{}
> { x:1, y:"why not", z: function() {} }
{ x: 1, y: 'why not', z: [Function] }
>
```
For the ambiguous inputs like `{ x }`, the existing REPL
behaviour (ES6 literal shorthand) is preserved (prefers
expression over statement).
Fixes: #5576
PR-URL: #5581
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

Author: princejwesley

lib/repl.js

@@ -220,7 +220,7 @@ function REPLServer(prompt,
 eval_=eval_||defaultEval;
 
 functiondefaultEval(code,context,file,cb){
-varerr,result,retry=false;
+varerr,result,retry=false,input=code,wrappedErr;
 // first, create the Script object to check the syntax
 while(true){
 try{
@@ -238,14 +238,23 @@ function REPLServer(prompt,
 debug('parse error %j',code,e);
 if(self.replMode===exports.REPL_MODE_MAGIC&&
 e.message===BLOCK_SCOPED_ERROR&&
-!retry){
-retry=true;
+!retry||self.wrappedCmd){
+if(self.wrappedCmd){
+self.wrappedCmd=false;
+// unwrap and try again
+code=`${input.substring(1,input.length-2)}\n`;
+wrappedErr=e;
+}else{
+retry=true;
+}
 continue;
 }
-if(isRecoverableError(e,self))
-err=newRecoverable(e);
+// preserve original error for wrapped command
+consterror=wrappedErr||e;
+if(isRecoverableError(error,self))
+err=newRecoverable(error);
 else
-err=e;
+err=error;
 }
 break;
 }
@@ -418,6 +427,7 @@ function REPLServer(prompt,
 // to wrap it in parentheses, so that it will be interpreted as
 // an expression.
 evalCmd='('+evalCmd+')\n';
+self.wrappedCmd=true;
 }else{
 // otherwise we just append a \n so that it will be either
 // terminated, or continued onto the next expression if it's an
@@ -435,6 +445,7 @@ function REPLServer(prompt,
 debug('finish',e,ret);
 self.memory(cmd);
 
+self.wrappedCmd=false;
 if(e&&!self.bufferedCommand&&cmd.trim().match(/^npm/)){
 self.outputStream.write('npm should be run outside of the '+
 'node repl, in your normal shell.\n'+

test/parallel/test-repl.js

@@ -323,6 +323,8 @@ function error_test() {
 {client: client_unix,send: 'function x(s) {\nreturn s.replace(/.*/,"");\n}',
 expect: prompt_multiline+prompt_multiline+
 'undefined\n'+prompt_unix},
+{client: client_unix,send: '{ var x = 4; }',
+expect: 'undefined\n'+prompt_unix},
 ]);
 }