Another stab at fixing CI flakiness

See #1243 for context. This is basically a more aggressive version of
pull #1230 -- instead of just running unit tests in isolation, we also
run flow in isolation, and kill the servers afterwards.

Test plan: See how this fares in CI :)

config/test.js

@@ -1,5 +1,7 @@
 // @flow
 
+/*:: import type {Task} from "../src/tools/execDependencyGraph"; */
+
 const tmp = require("tmp");
 
 const execDependencyGraph = require("../src/tools/execDependencyGraph");
@@ -146,18 +148,39 @@ function makeTasks(
   })();
   if (limitMemoryUsage) {
     // We've had issues with our tests flakily failing in CI, due to apparent
-    // memory issues. I've found that if we both limit the maxWorkers to 2, and
-    // ensure that nothing else runs at the same time as jest, then it stops
-    // flakily failing.
+    // memory issues.
+    //
+    // This block attempts to limit memory usage by having flow to run first,
+    // then stopping the flow server, then running unit tests, and only
+    // afterwards running all other tasks.
+    //
+    // The reasoning is that the flow server is fairly memory demanding and we
+    // can safely kill it after we've checked the types, and jest is also quite
+    // memory intensive. Hopefully by finishing these tasks first and releasing
+    // their resources, we won't have more memory exhaustion.
     tasks.forEach((task) => {
-      if (task.id === "unit") {
-        task.cmd.push("--maxWorkers=2");
-      } else {
-        // Ensure that everything else depends on unit tests, so unit tests
-        // will run first and run alone.
-        task.deps.push("unit");
+      switch (task.id) {
+        case "flow":
+          // Run flow first
+          return;
+        case "unit":
+          task.cmd.push("--maxWorkers=2");
+          // Run unit after we _stopped_ the flow server
+          // (to free up memory from flow)
+          task.deps.push("flow-stop");
+          return;
+        default:
+          // Run everything else after unit tests
+          // (unit is a memory hog)
+          task.deps.push("unit");
       }
     });
+    const flowStopTask /*: Task */ = {
+      id: "flow-stop",
+      cmd: ["yarn", "run", "--silent", "flow", "stop"],
+      deps: ["flow"],
+    };
+    tasks.push(flowStopTask);
   }
   return tasks;
 }

src/tools/execDependencyGraph.js

@@ -9,21 +9,21 @@ const chalk = require("chalk");
 const child_process = require("child_process");
 
 /*::
-type TaskId = string;
-type Task = {|
+export type TaskId = string;
+export type Task = {|
   +id: TaskId,
   +cmd: $ReadOnlyArray<string>,
   +deps: $ReadOnlyArray<TaskId>,
 |};
 
-type TaskResult = {|
+export type TaskResult = {|
   +id: TaskId,
   +success: boolean,
   +status: number,
   +stdout: string,
   +stderr: string,
 |};
-type OverallResult = {|
+export type OverallResult = {|
   +success: boolean,
 |};
 
@@ -31,7 +31,7 @@ type OverallResult = {|
 // should all be of the same width. The default options use a
 // 4-character string for each. Shorter strings with length of the same
 // parity can be extended by symmetrically adding spaces.
-type RunOptions = {|
+export type RunOptions = {|
   +taskPassLabel?: string,
   +taskFailLabel?: string,
   +taskLaunchLabel?: string,