src/vs/base/browser/dom.ts/SizeUtils/getDimension
class SizeUtils:	private static getDimension(element: HTMLElement, cssPropertyName: string, jsPropertyName: string): number {
		let computedStyle: CSSStyleDeclaration = getComputedStyle(element);
		let value = '0';
		if (computedStyle) {
			if (computedStyle.getPropertyValue) {
				value = computedStyle.getPropertyValue(cssPropertyName);
			} else {
				// IE8
				value = (<any>computedStyle).getAttribute(jsPropertyName);
			}
		}
		return SizeUtils.convertToPixels(element, value);
	}