Make test Context an ES6 module (#7142)

* Make test Context an ES6 module

* Address comments

karma.conf.js

@@ -68,7 +68,7 @@ module.exports = function(karma) {
 			plugins: [
 				resolve(),
 				babel({exclude: 'node_modules/**'}), // use babel since we have ES proposal features
-				commonjs()
+				commonjs({exclude: ['src/**', 'test/**']})
 			],
 			output: {
 				name: 'test',

test/context.js

@@ -1,5 +1,5 @@
 // Code from https://stackoverflow.com/questions/4406864/html-canvas-unit-testing
-var Context = function() {
+const Context = function() {
 	this._calls = []; // names/args of recorded calls
 	this._initMethods();
 
@@ -136,4 +136,4 @@ Context.prototype.resetCalls = function() {
 	this._calls = [];
 };
 
-module.exports = Context;
+export default Context;