Clean up ProjectFileInvalidator.findInvalidated a bit (#41889)

In preparation for some refactoring that I will be doing to
`ProjectFileInvalidator.findInvalidated`, make its code a bit
clearer:
* Indicate which arguments may be null.
* Don't bother calling `FileStat.statSync` on the `.packages` file
  for the initial load.  This makes the checks for the `.packages`
  file consistent with those for other files.
* Use `DateTime.isAfter()` instead of comparing microseconds
  ourselves.

While I was touching this file, I also removed some unnecessary
comparisons to `false`. (`_ManifestAssetBundle.wasBuiltOnce()` is the
only implementation I can find of `AssetBundle.wasBuiltOnce()`, and
it never returns `null`.)

packages/flutter_tools/lib/src/run_hot.dart

@@ -283,7 +283,7 @@ class HotRunner extends ResidentRunner {
   }
 
   Future<UpdateFSReport> _updateDevFS({ bool fullRestart = false }) async {
-    final bool isFirstUpload = assetBundle.wasBuiltOnce() == false;
+    final bool isFirstUpload = !assetBundle.wasBuiltOnce();
     final bool rebuildBundle = assetBundle.needsBuild();
     if (rebuildBundle) {
       printTrace('Updating assets');
@@ -308,7 +308,7 @@ class HotRunner extends ResidentRunner {
         bundle: assetBundle,
         firstBuildTime: firstBuildTime,
         bundleFirstUpload: isFirstUpload,
-        bundleDirty: isFirstUpload == false && rebuildBundle,
+        bundleDirty: !isFirstUpload && rebuildBundle,
         fullRestart: fullRestart,
         projectRootPath: projectRootPath,
         pathToReload: getReloadPath(fullRestart: fullRestart),
@@ -1045,6 +1045,14 @@ class ProjectFileInvalidator {
     @required List<Uri> urisToMonitor,
     @required String packagesPath,
   }) {
+    assert(urisToMonitor != null);
+    assert(packagesPath != null);
+
+    if (lastCompiled == null) {
+      assert(urisToMonitor.isEmpty);
+      return <Uri>[];
+    }
+
     final List<Uri> invalidatedFiles = <Uri>[];
     int scanned = 0;
     final Stopwatch stopwatch = Stopwatch()..start();
@@ -1057,17 +1065,13 @@ class ProjectFileInvalidator {
       final DateTime updatedAt = fs.statSync(
           uri.toFilePath(windows: platform.isWindows)).modified;
       scanned++;
-      if (updatedAt == null) {
-        continue;
-      }
-      if (updatedAt.millisecondsSinceEpoch > lastCompiled.millisecondsSinceEpoch) {
+      if (updatedAt != null && updatedAt.isAfter(lastCompiled)) {
         invalidatedFiles.add(uri);
       }
     }
-    // we need to check the .packages file too since it is not used in compilation.
+    // We need to check the .packages file too since it is not used in compilation.
     final DateTime packagesUpdatedAt = fs.statSync(packagesPath).modified;
-    if (lastCompiled != null && packagesUpdatedAt != null
-        && packagesUpdatedAt.millisecondsSinceEpoch > lastCompiled.millisecondsSinceEpoch) {
+    if (packagesUpdatedAt != null && packagesUpdatedAt.isAfter(lastCompiled)) {
       invalidatedFiles.add(fs.file(packagesPath).uri);
       scanned++;
     }

packages/flutter_tools/test/general.shard/project_file_invalidator_test.dart

@@ -12,6 +12,12 @@ import '../src/context.dart';
 void main() {
   group('ProjectFileInvalidator', () {
     final MemoryFileSystem memoryFileSystem = MemoryFileSystem();
+    testUsingContext('No last compile', () async {
+      expect(
+        ProjectFileInvalidator.findInvalidated(lastCompiled: null, urisToMonitor: <Uri>[], packagesPath: ''),
+        isEmpty);
+    });
+
     testUsingContext('Empty project', () async {
       expect(
         ProjectFileInvalidator.findInvalidated(lastCompiled: DateTime.now(), urisToMonitor: <Uri>[], packagesPath: ''),