Migration: Make the three migration phases explicit in NonNullableFix API.

Previously the API was general, exposing a `numPhases` integer, and
providing a generic processing function with a `phase` argument.  This
was necessary back when migration was built into DartFix (because
different kinds of fixes had different numbers of phases), but now
that migration is a separate tool, it gets in the way because it makes
it difficult for the caller to have different behaviors in different
phases.

This is a prerequisite for making it possible to have some files
analyzed but not finalized (which is in turn necessary for handling
generated files).

Change-Id: I5993808eb3dace21d8b6cb9c5308f94a3b1f898e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151081
Reviewed-by: Samuel Rawlins <[email protected]>

Author: stereotype441

pkg/nnbd_migration/lib/migration_cli.dart

@@ -848,6 +848,8 @@ class _BadArgException implements Exception {
 }
 
 class _FixCodeProcessor extends Object {
+  static const numPhases = 3;
+
   final DriverBasedAnalysisContext context;
 
   NonNullableFix _task;
@@ -922,7 +924,7 @@ class _FixCodeProcessor extends Object {
   Future<void> runFirstPhase({bool singlePhaseProgress = false}) async {
     // All tasks should be registered; [numPhases] should be finalized.
     _progressBar = _ProgressBar(
-        pathsToProcess.length * (singlePhaseProgress ? 1 : _task.numPhases));
+        pathsToProcess.length * (singlePhaseProgress ? 1 : numPhases));
 
     // Process package
     _task.processPackage(context.contextRoot.root);
@@ -939,23 +941,24 @@ class _FixCodeProcessor extends Object {
       }
       if (_migrationCli.options.ignoreErrors ||
           _migrationCli.fileErrors.isEmpty) {
-        await _task.processUnit(0, result);
+        await _task.prepareUnit(result);
       }
     });
   }
 
   Future<List<String>> runLaterPhases({bool resetProgress = false}) async {
     if (resetProgress) {
-      _progressBar =
-          _ProgressBar(pathsToProcess.length * (_task.numPhases - 1));
+      _progressBar = _ProgressBar(pathsToProcess.length * (numPhases - 1));
     }
 
-    for (var phase = 1; phase < _task.numPhases; phase++) {
-      await processResources((ResolvedUnitResult result) async {
-        _progressBar.tick();
-        await _task.processUnit(phase, result);
-      });
-    }
+    await processResources((ResolvedUnitResult result) async {
+      _progressBar.tick();
+      await _task.processUnit(result);
+    });
+    await processResources((ResolvedUnitResult result) async {
+      _progressBar.tick();
+      await _task.finalizeUnit(result);
+    });
     var state = await _task.finish();
     if (_migrationCli.options.webPreview) {
       await _task.startPreviewServer(state);

pkg/nnbd_migration/lib/src/front_end/non_nullable_fix.dart

@@ -91,12 +91,17 @@ class NonNullableFix {
 
   bool get isPreviewServerRunning => _server != null;
 
-  int get numPhases => 3;
-
   InstrumentationListener createInstrumentationListener(
           {MigrationSummary migrationSummary}) =>
       InstrumentationListener(migrationSummary: migrationSummary);
 
+  Future<void> finalizeUnit(ResolvedUnitResult result) async {
+    if (!_packageIsNNBD) {
+      return;
+    }
+    migration.finalizeInput(result);
+  }
+
   Future<MigrationState> finish() async {
     migration.finish();
     final state = MigrationState(
@@ -105,6 +110,13 @@ class NonNullableFix {
     return state;
   }
 
+  Future<void> prepareUnit(ResolvedUnitResult result) async {
+    if (!_packageIsNNBD) {
+      return;
+    }
+    migration.prepareInput(result);
+  }
+
   /// Processes the non-source files of the package rooted at [pkgFolder].
   ///
   /// This means updating the pubspec.yaml file and the package_config.json
@@ -138,23 +150,11 @@ class NonNullableFix {
     }
   }
 
-  Future<void> processUnit(int phase, ResolvedUnitResult result) async {
+  Future<void> processUnit(ResolvedUnitResult result) async {
     if (!_packageIsNNBD) {
       return;
     }
-    switch (phase) {
-      case 0:
-        migration.prepareInput(result);
-        break;
-      case 1:
-        migration.processInput(result);
-        break;
-      case 2:
-        migration.finalizeInput(result);
-        break;
-      default:
-        throw ArgumentError('Unsupported phase $phase');
-    }
+    migration.processInput(result);
   }
 
   Future<MigrationState> rerun() async {