Pass require() as an externs def to the JS compiler.

Bug: Issue 10653
Change-Id: I2bb56a8069e7a9f6f8b323f7cae62de948db9a44

BUILD

diff --git a/BUILD b/BUILD
index f84f778..4733856 100644
--- a/BUILD
+++ b/BUILD
@@ -3,6 +3,7 @@
 polygerrit_plugin(
     name = "image-diff",
     srcs = glob(["**/*.html", "**/*.js"]),
+    externs = ["externs.js"],
     app = "plugin.html",
     deps = ["//lib/js:resemblejs"],
-)
\ No newline at end of file
+)

externs.js

diff --git a/externs.js b/externs.js
new file mode 100644
index 0000000..2d41eb5
--- /dev/null
+++ b/externs.js
@@ -0,0 +1,14 @@
+/**
+ * @fileoverview Defines externally defined symbols that the JS Closure compiler
+ * must be aware of: 1. Don't complain about "is undeclared". 2. Don't rename.
+ *
+ * @externs
+ */
+
+/**
+ * resemblejs uses the nodejs require fuction without declaring it
+ *
+ * @param {string} id
+ * @return {*}
+ */
+function require(id) {}