Index: Gruntfile.js
===================================================================
--- Gruntfile.js	(revision 50536)
+++ Gruntfile.js	(working copy)
@@ -1126,6 +1126,26 @@
 						dest: BUILD_DIR + 'wp-includes/'
 					}
 				]
+			},
+			underscoreSourceMap: {
+				options: {
+					patterns: [
+						{
+							match: new RegExp( '//# sourceMappingURL=.*\\s*' ),
+							replacement: ''
+						}
+					]
+				},
+				files: [
+					{
+						expand: true,
+						flatten: true,
+						src: [
+							BUILD_DIR + 'wp-includes/js/underscore.js'
+						],
+						dest: BUILD_DIR + 'wp-includes/js/'
+					}
+				]
 			}
 		},
 		_watch: {
@@ -1569,6 +1589,7 @@
 				'includes:emoji',
 				'includes:embed',
 				'replace:emojiBannerText',
+				'replace:underscoreSourceMap',
 				'verify:build'
 			] );
 		}
Index: package-lock.json
===================================================================
--- package-lock.json	(revision 50536)
+++ package-lock.json	(working copy)
@@ -5146,6 +5146,13 @@
 			"integrity": "sha512-RLmDrRXkVdouTg38jcgHhyQ/2zjg7a8E6sz2zxfz21Hh17xDJYUHBZimVIt5fUyS8vbfpeSmTL3gUjTEvUV3qQ==",
 			"requires": {
 				"underscore": ">=1.8.3"
+			},
+			"dependencies": {
+				"underscore": {
+					"version": "1.12.1",
+					"resolved": "https://registry.npmjs.org/underscore/-/underscore-1.12.1.tgz",
+					"integrity": "sha512-hEQt0+ZLDVUMhebKxL4x1BTtDY7bavVofhZ9KZ4aI26X9SRaE+Y3m83XUL1UP2jn8ynjndwCCpEHdUG+9pP1Tw=="
+				}
 			}
 		},
 		"bail": {
@@ -5747,7 +5754,7 @@
 		},
 		"browserify-aes": {
 			"version": "1.2.0",
-			"resolved": "http://registry.npmjs.org/browserify-aes/-/browserify-aes-1.2.0.tgz",
+			"resolved": "https://registry.npmjs.org/browserify-aes/-/browserify-aes-1.2.0.tgz",
 			"integrity": "sha512-+7CHXqGuspUn/Sl5aO7Ea0xWGAtETPXNSAjHo48JfLdPWcMng33Xe4znFvQweqc/uzk5zSOI3H52CYnjCfb5hA==",
 			"dev": true,
 			"requires": {
@@ -22853,9 +22860,9 @@
 			"dev": true
 		},
 		"underscore": {
-			"version": "1.8.3",
-			"resolved": "https://registry.npmjs.org/underscore/-/underscore-1.8.3.tgz",
-			"integrity": "sha1-Tz+1OxBuYJf8+ctBCfKl6b36UCI="
+			"version": "1.13.0-1",
+			"resolved": "https://registry.npmjs.org/underscore/-/underscore-1.13.0-1.tgz",
+			"integrity": "sha512-0jWW2h33TcKtPDORwBZW2g20yTUYzfRLS9D7LvXEAigJjFjpYmQzrTphauffz9kEtjMrPcjNaStHAHJwd5lexw=="
 		},
 		"underscore.string": {
 			"version": "3.3.5",
Index: package.json
===================================================================
--- package.json	(revision 50536)
+++ package.json	(working copy)
@@ -146,7 +146,7 @@
 		"react": "16.13.1",
 		"react-dom": "16.13.1",
 		"twemoji": "13.0.1",
-		"underscore": "1.8.3",
+		"underscore": "1.13.0-1",
 		"whatwg-fetch": "3.0.0"
 	},
 	"scripts": {