diff --git a/tasks/test_syntax.js b/tasks/test_syntax.js
index f4591496238..150cac24f4c 100644
--- a/tasks/test_syntax.js
+++ b/tasks/test_syntax.js
@@ -5,6 +5,7 @@ var falafel = require('falafel');
 var glob = require('glob');
 var madge = require('madge');
 var readLastLines = require('read-last-lines');
+var eslint = require('eslint');
 
 var constants = require('./util/constants');
 var srcGlob = path.join(constants.pathToSrc, '**/*.js');
@@ -20,6 +21,7 @@ assertSrcContents();
 assertFileNames();
 assertTrailingNewLine();
 assertCircularDeps();
+assertES5();
 
 
 // check for for focus and exclude jasmine blocks
@@ -188,6 +190,38 @@ function assertCircularDeps() {
     });
 }
 
+// Ensure no ES6 has snuck through into the build:
+function assertES5() {
+    var CLIEngine = eslint.CLIEngine;
+
+    var cli = new CLIEngine({
+        useEslintrc: false,
+        ignore: false,
+        parserOptions: {
+            ecmaVersion: 5
+        }
+    });
+
+    var files = constants.partialBundlePaths.map(function(f) { return f.dist; });
+    files.unshift(constants.pathToPlotlyDist);
+
+    var report = cli.executeOnFiles(files);
+    var formatter = cli.getFormatter();
+
+    var errors = [];
+    if(report.errorCount > 0) {
+        console.log(formatter(report.results));
+
+        // It doesn't work well to pass formatted logs into this,
+        // so instead pass the empty string in a way that causes
+        // the test to fail
+        errors.push('');
+    }
+
+    log('es5-only syntax', errors);
+}
+
+
 function combineGlobs(arr) {
     return '{' + arr.join(',') + '}';
 }