Fix link + docs (#2476)

* Fix a copy/paste fail

Also, fix some incorrect tests.

* Clarify how routes are diffed, improve key + route resolver docs

- Add some missing links to route resolvers and single-child keyed
  fragments, clarify usage around them.
- Drive-by: remove a redundant sentence that itself was missing a
  period.

* Actually test for propagation and preventDefault

Previously, the mocks were both junk and inaccurate. No wonder my tests
were silently failing - they were wrong and not obviously wrong.

test-utils/domMock.js

@@ -401,7 +401,7 @@ module.exports = function(options) {
 						e.preventDefault = function() {
 							prevented = true
 						}
-						Object.defineProperty(e, "$defaultPrevented", {
+						Object.defineProperty(e, "defaultPrevented", {
 							configurable: true,
 							get: function () { return prevented }
 						})
@@ -409,10 +409,6 @@ module.exports = function(options) {
 						e.stopPropagation = function() {
 							stopped = true
 						}
-						Object.defineProperty(e, "$propagationStopped", {
-							configurable: true,
-							get: function () { return prevented }
-						})
 						e.eventPhase = 1
 						try {
 							for (var i = parents.length - 1; 0 <= i; i--) {