diff --git a/.travis.yml b/.travis.yml
index 2223daadff..6e68b9c382 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,5 +1,12 @@
 language: node_js
 after_success: npm run coverage && cat ./coverage/lcov.info | coveralls
+before_install:
+  # These packages cannot be installed in legacy versions of Node.js. Because
+  # they are only needed to verify correctness in a browser-like environment,
+  # they can be removed (and the tests skipped) without effecting coverage.
+  - if [[ $(node --version) != v6* ]] ; then
+      node scripts/remove-dev-dependencies.js phantom phantomjs-prebuilt;
+    fi
 script:
   - npm run pretest
   - npm run test-node
diff --git a/appveyor.yml b/appveyor.yml
index d338bfc210..7ee4f33693 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -12,6 +12,10 @@ environment:
 install:
   - ps: Install-Product node $env:nodejs_version
   - git submodule update --init --recursive
+  # These packages cannot be installed in legacy versions of Node.js. Because
+  # they are only needed to verify correctness in a browser-like environment,
+  # they can be removed (and the tests skipped) without effecting coverage.
+  - if NOT "%nodejs_version%" == "6" node scripts/remove-dev-dependencies.js phantom phantomjs-prebuilt
   - npm install
 
 build: off
diff --git a/package.json b/package.json
index d926055486..7e57ab3fe9 100644
--- a/package.json
+++ b/package.json
@@ -59,16 +59,14 @@
     "jscs": "1.11.x",
     "mock-stdin": "0.3.x",
     "nodeunit": "0.9.x",
+    "phantom": "~4.0.1",
+    "phantomjs-prebuilt": "~2.1.7",
     "regenerate": "1.2.x",
     "results-interpreter": "~1.0.0",
     "sinon": "1.12.x",
     "test262-stream": "~1.1.0",
     "unicode-11.0.0": "0.7.x"
   },
-  "optionalDependencies": {
-    "phantom": "~4.0.1",
-    "phantomjs-prebuilt": "~2.1.7"
-  },
   "license": "(MIT AND JSON)",
   "preferGlobal": true,
   "files": [
diff --git a/scripts/remove-dev-dependencies.js b/scripts/remove-dev-dependencies.js
new file mode 100644
index 0000000000..7b5149b543
--- /dev/null
+++ b/scripts/remove-dev-dependencies.js
@@ -0,0 +1,37 @@
+"use strict"
+/**
+ * Some of this project's development dependencies cannot be successfully
+ * installed in legacy versions of Node.js. This module removes development
+ * dependencies from the project's npm package manifest so that the `npm`
+ * utility does not attempt to install them in a subsequent execution of `npm
+ * install. Although the `npm` utility offers a "remove" command, that command
+ * can remove only one package per invocation and triggers installation of all
+ * other packages. This script allows mutliple packages to be removed prior to
+ * installation.
+ */
+
+var path = require("path");
+var fs = require("fs");
+var manifestLocation = path.join(__dirname, "..", "package.json");
+var manifest = require(manifestLocation);
+var packageNames = process.argv.slice(2);
+var newContents;
+
+packageNames.forEach(function(packageName) {
+  if (!manifest.devDependencies[packageName]) {
+    throw new Error("Could not locate development dependency named \"" + packageName + "\"");
+  }
+
+  delete manifest.devDependencies[packageName];
+});
+
+newContents = JSON.stringify(manifest, null, 2);
+
+fs.writeFile(manifestLocation, newContents, function(err) {
+  if (err) {
+    throw new Error(err);
+  }
+
+  console.log("Successfully removed packages. New contents:");
+  console.log(newContents.replace(/(^|\n)/g, "$1> "));
+});