Extract `dedent` to `util`, adding tests (#538)

Summary:
There have been a couple of occasions on which we’ve considered using
it, but didn’t want to require from `app/`.

Test Plan:
Unit tests added, with full coverage.

wchargin-branch: extract-dedent

src/app/server.js

@@ -7,7 +7,7 @@ import {match, RouterContext} from "react-router";
 
 import {createRoutes} from "./createRoutes";
 import {resolveTitleFromPath} from "./routeData";
-import dedent from "./dedent";
+import dedent from "../util/dedent";
 
 export default function render(
   locals: {+path: string, +assets: {[string]: string}},

src/util/dedent.test.js

@@ -0,0 +1,47 @@
+// @flow
+
+import dedent from "./dedent";
+
+describe("util/dedent", () => {
+  it("dedents a simple example", () => {
+    const actual = dedent`\
+      hello
+        good
+      world
+    `;
+    const expected = "hello\n  good\nworld\n";
+    expect(actual).toEqual(expected);
+  });
+
+  it("interpolates components", () => {
+    const ell = "l";
+    const actual = dedent`\
+      he${ell}${ell}o
+        good
+      wor${ell}d
+    `;
+    const expected = "hello\n  good\nworld\n";
+    expect(actual).toEqual(expected);
+  });
+
+  it("does not strip leading whitespace in components", () => {
+    // See: https://github.com/wchargin/wchargin.github.io/commit/06475d4cc44a0437c911dd2d4d6275be4381142e
+    const code = 'if (true) {\n  console.log("hi");\n}';
+    const actual = dedent`\
+    <pre><code>${code}</code></pre>
+    `;
+    const expected = `<pre><code>${code}</code></pre>\n`;
+    expect(actual).toEqual(expected);
+  });
+
+  it("does not strip trailing backslashes", () => {
+    // See: https://github.com/wchargin/wchargin.github.io/commit/06475d4cc44a0437c911dd2d4d6275be4381142e
+    const code = "printf '%s' \\\n wat";
+    const actual = dedent`\
+    $ cat foo.sh
+    ${code}
+    `;
+    const expected = `$ cat foo.sh\n${code}\n`;
+    expect(actual).toEqual(expected);
+  });
+});