Request.formData() is not implemented in Chrome, so here's a polyfill that will sort it out

Request.formData.js

// OK, Chrome (and all browsers apart from Firefox) don't support Request.formData(). So, we have to polyfill it
if (!Request.formData) {
	Request.prototype.formData = function() {
		return this.text().then(text => {
			const fd = new FormData(),
				contentType = this.headers.get('content-type');
			// The text here may be a URIEncoded string, rather than multipart form
			// Two different decoding strategies are needed
			if (contentType.startsWith('multipart/form-data;')) {
				// multipart/form-data encoding strategy
				const boundaryKey = 'boundary=',
					boundary = contentType.substr(contentType.indexOf(boundaryKey)+boundaryKey.length),
					lines = text.split('\r\n');

				for (let i = 0; i < lines.length; i++) {
					let line = lines[i];
					// Spec says lines start with '--' and then the boundary in the header
					// https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
					if ((line.startsWith('--'+boundary)) && i + 3 < lines.length) {
						fd.append(lines[i + 1].match(/\"(.*)\"/)[1], lines[i + 3]);
					}
				}
			} else if(contentType.startsWith('application/x-www-form-urlencoded;')) {
				// Should be application/x-www-form-urlencoded. Use URLSearchParams for data
				const params = new URLSearchParams(text);
				for (const [key, val] of params) {
					fd.append(key,val);
				}
			} else {
				// There's also text/plain encoding. No strategy for that right now
				throw TypeError(`Encoding ${contentType} not supported`);
			}
			
			return fd;
		});
	};
} 

Updated gist with my latest version - comments updated, logs removed.

Thanks for the help Jake