repl: treat "Assignment to const" as syntax error

Adding a new `repl-harmony` test file here because adding the
`--use_strict --harmony` flags on the main repl test file was causing
lots of unrelated failures, due to global variable assignments and
things like that. This new test file is based off of the original
repl.js test file, but has a lot of the tests stripped out. A test case
for this commit is included though.

Fixes #6132.

lib/repl.js

@@ -949,5 +949,6 @@ function isSyntaxError(e) {
   e = e && (e.stack || e.toString());
   return e && e.match(/^SyntaxError/) &&
       // "strict mode" syntax errors
-      !e.match(/^SyntaxError: .*strict mode.*/i);
+      !e.match(/^SyntaxError: .*strict mode.*/i) &&
+      !e.match(/^SyntaxError: Assignment to constant variable/i);
 }

test/simple/test-repl-harmony.js

@@ -0,0 +1,47 @@
+// Copyright Joyent, Inc. and other Node contributors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to permit
+// persons to whom the Software is furnished to do so, subject to the
+// following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+// NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+// DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+// USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+var common = require('../common');
+var assert = require('assert');
+
+var spawn = require('child_process').spawn;
+var args = ['--harmony', '--use-strict', '-i'];
+var child = spawn(process.execPath, args);
+
+var input = 'function x(){const y=1;y=2}\n';
+var expectOut = /^> SyntaxError: Assignment to constant variable.\n/;
+
+child.stderr.setEncoding('utf8');
+child.stderr.on('data', function(c) {
+  throw new Error('child.stderr be silent');
+});
+
+child.stdout.setEncoding('utf8');
+var out = '';
+child.stdout.on('data', function(c) {
+  out += c;
+});
+child.stdout.on('end', function() {
+  assert(expectOut.test(out));
+  console.log('ok');
+});
+
+child.stdin.end(input);