diff --git a/.eslintrc.json b/.eslintrc.json
index e474d8566..3df051d4a 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -74,7 +74,8 @@
                 "balanced": true
             },
             "exceptions": ["*"]
-        }]
+        }],
+        "es5/no-es6-methods": ["warn"]
     },
     "env": {
         "browser": true
@@ -83,5 +84,8 @@
         "Blockly": true,
         "goog": true
     },
-    "extends": "eslint:recommended"
+    "extends": [
+        "eslint:recommended",
+        "plugin:es5/no-es2015"
+    ]
 }
diff --git a/core/utils/object.js b/core/utils/object.js
index f8b6a55ba..2a9b848a9 100644
--- a/core/utils/object.js
+++ b/core/utils/object.js
@@ -55,7 +55,9 @@ Blockly.utils.object.mixin = function(target, source) {
  */
 Blockly.utils.object.values = function(obj) {
   if (Object.values) {
+    /* eslint-disable es5/no-es6-methods */
     return Object.values(obj);
+    /* eslint-enable es5/no-es6-methods */
   }
   // Fallback for IE.
   return Object.keys(obj).map(function(e) {
diff --git a/gulpfile.js b/gulpfile.js
index ce232b7c0..8959fbc42 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -88,6 +88,7 @@ function compile(compilerOptions, opt_verbose) {
   if (!compilerOptions) compilerOptions = {};
   compilerOptions.compilation_level = 'SIMPLE_OPTIMIZATIONS';
   compilerOptions.warning_level = opt_verbose ? 'VERBOSE' : 'DEFAULT';
+  compilerOptions.language_in = 'ECMASCRIPT5_STRICT';
   compilerOptions.language_out = 'ECMASCRIPT5_STRICT';
   compilerOptions.rewrite_polyfills = false;
   compilerOptions.hide_warnings_for = 'node_modules';
diff --git a/package-lock.json b/package-lock.json
index e149f20aa..fcf27e0d6 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1612,6 +1612,12 @@
         "text-table": "^0.2.0"
       }
     },
+    "eslint-plugin-es5": {
+      "version": "1.4.1",
+      "resolved": "https://registry.npmjs.org/eslint-plugin-es5/-/eslint-plugin-es5-1.4.1.tgz",
+      "integrity": "sha512-kktkmkF2O7pnSZYgrMiYMbt3wCKRIiXePwILv8USDG95YgP0PzhIxSIROLLKmiQQ/Z6LuhDGWTHK04gnbXBvkg==",
+      "dev": true
+    },
     "eslint-scope": {
       "version": "4.0.3",
       "resolved": "https://registry.npmjs.org/eslint-scope/-/eslint-scope-4.0.3.tgz",
diff --git a/package.json b/package.json
index 612470f57..d03df46a2 100644
--- a/package.json
+++ b/package.json
@@ -48,6 +48,7 @@
     "chai": "^4.2.0",
     "concurrently": "^4.1.2",
     "eslint": "^5.13.0",
+    "eslint-plugin-es5": "^1.4.1",
     "fs": "0.0.1-security",
     "google-closure-compiler": "^20190618.0.0",
     "google-closure-library": "^20190618.0.0",
diff --git a/tests/compile/compile.sh b/tests/compile/compile.sh
index 94d1e5b18..66486303c 100755
--- a/tests/compile/compile.sh
+++ b/tests/compile/compile.sh
@@ -80,6 +80,8 @@ COMPILATION_COMMAND="java -jar $COMPILER --js='$BLOCKLY_ROOT/tests/compile/main.
   --generate_exports \
   --externs $BLOCKLY_ROOT/externs/svg-externs.js \
   --compilation_level ADVANCED_OPTIMIZATIONS \
+  --language_in ECMASCRIPT5_STRICT \
+  --language_out ECMASCRIPT5_STRICT \
   --dependency_mode=STRICT --entry_point=Main \
   --js_output_file $BLOCKLY_ROOT/tests/compile/main_compressed.js"
 echo "$COMPILATION_COMMAND"
diff --git a/tests/node/run_node_test.js b/tests/node/run_node_test.js
index 7e2530662..2c3890348 100644
--- a/tests/node/run_node_test.js
+++ b/tests/node/run_node_test.js
@@ -22,28 +22,28 @@
 var assert = require('chai').assert;
 var Blockly = require('../../dist/');
 
-var xmlText = `<xml xmlns="https://developers.google.com/blockly/xml">
-  <block type="text_print" x="37" y="63">
-    <value name="TEXT">
-      <shadow type="text">
-        <field name="TEXT">Hello from Blockly!</field>
-      </shadow>
-    </value>
-  </block>
-</xml>`;
+var xmlText = '<xml xmlns="https://developers.google.com/blockly/xml">\n' +
+'  <block type="text_print" x="37" y="63">\n' +
+'    <value name="TEXT">\n' +
+'      <shadow type="text">\n' +
+'        <field name="TEXT">Hello from Blockly!</field>\n' +
+'      </shadow>\n' +
+'    </value>\n' +
+'  </block>\n' +
+'</xml>';
 
 suite('Test Node.js', function() {
   test('Import XML', function() {
-    const xml = Blockly.Xml.textToDom(xmlText);
+    var xml = Blockly.Xml.textToDom(xmlText);
 
     // Create workspace and import the XML
-    const workspace = new Blockly.Workspace();
+    var workspace = new Blockly.Workspace();
     Blockly.Xml.domToWorkspace(xml, workspace);
   });
   test('Roundtrip XML', function() {
-    const xml = Blockly.Xml.textToDom(xmlText);
+    var xml = Blockly.Xml.textToDom(xmlText);
 
-    const workspace = new Blockly.Workspace();
+    var workspace = new Blockly.Workspace();
     Blockly.Xml.domToWorkspace(xml, workspace);
 
     var headlessXml = Blockly.Xml.workspaceToDom(workspace, true);
@@ -52,17 +52,17 @@ suite('Test Node.js', function() {
     assert.equal(headlessText, xmlText, 'equal');
   });
   test('Generate Code', function() {
-    const xml = Blockly.Xml.textToDom(xmlText);
+    var xml = Blockly.Xml.textToDom(xmlText);
 
     // Create workspace and import the XML
-    const workspace = new Blockly.Workspace();
+    var workspace = new Blockly.Workspace();
     Blockly.Xml.domToWorkspace(xml, workspace);
 
     // Convert code
-    const code = Blockly.JavaScript.workspaceToCode(workspace);
+    var code = Blockly.JavaScript.workspaceToCode(workspace);
     
     // Check output
-    assert.equal(`window.alert('Hello from Blockly!');`, code.trim(), 'equal');
+    assert.equal('window.alert(\'Hello from Blockly!\');', code.trim(), 'equal');
   });
 });