link: verify that routes have trailing slash (#1001)

Summary:
This serves as a regression test for #1000.

Test Plan:
Note that `yarn unit` passes with this patch but fails if the change to
the code is reverted, or if the patch in #1000 is reverted. Note that
`yarn build` also passes but fails if the patch in #1000 is reverted.
Note also that `yarn test --full` passes.

wchargin-branch: link-verify-trailing-slash

src/webutil/Link.js

@@ -41,6 +41,10 @@ export default class Link extends Component<LinkProps> {
       </Tag>
     );
     if ("to" in this.props) {
+      if (!this.props.to.endsWith("/")) {
+        // All our routes have trailing slashes. This must be an error.
+        throw new Error("'to' prop must specify route with trailing slash.");
+      }
       return make(RouterLink);
     } else if ("href" in this.props) {
       return make("a");

src/webutil/Link.test.js

@@ -18,7 +18,7 @@ describe("webutil/Link", () => {
   void [
     // Must specify either `href` or `to`
     <Link href="https://example.com/">click me</Link>,
-    <Link to="/prototype">click me, too</Link>,
+    <Link to="/prototype/">click me, too</Link>,
     // $ExpectFlowError
     <Link>missing to/href</Link>,
 
@@ -38,9 +38,9 @@ describe("webutil/Link", () => {
   });
 
   it("renders a styled router link", () => {
-    const element = shallow(<Link to="/prototype">check it out</Link>);
+    const element = shallow(<Link to="/prototype/">check it out</Link>);
     expect(element.type()).toEqual(RouterLink);
-    expect(element.prop("to")).toEqual("/prototype");
+    expect(element.prop("to")).toEqual("/prototype/");
     expect(element.children().text()).toEqual("check it out");
     expect(typeof element.prop("className")).toBe("string");
   });
@@ -53,6 +53,20 @@ describe("webutil/Link", () => {
     }).toThrow("Must specify either 'to' or 'href'.");
   });
 
+  it("fails if router link omits a trailing slash", () => {
+    const component = <Link to="/foo" />;
+    expect(() => {
+      shallow(component);
+    }).toThrow("'to' prop must specify route with trailing slash.");
+  });
+
+  it("permits external link to omit a trailing slash", () => {
+    const component = <Link href="/foo" />;
+    expect(() => {
+      shallow(component);
+    }).not.toThrow();
+  });
+
   it("has deterministic className", () => {
     const e1 = shallow(<Link href="#" />);
     const e2 = shallow(<Link href="#" />);