Migration: Fix a corner case crash when removing all metadata nodes.

Previously, when removing all metadata nodes from code such as this:

    f({
      @removeMe
      int? i}) {}

If the node(s) being removed occupied entire lines (as they do in this
example), the logic for trying to decide whether to collapse together
empty `{}`, `[]`, or `()` would get confused by the fact that the text
being removed began at the start of the line, even though the parent
AST node (`@removeMe int? i`) didn't begin until partway through the
line; this would cause an assertion failure (or, if assertions were
disabled, an out-of-range error).

As of yet, this has never been a problem in practice, because the only
circumstance in which the migration engine removes metadata is when
adding other metadata (and thus the logic for trying to collapse
together empty `{}`, `[]` or `()` doesn't trigger).  But in a future
CL I'll be adding logic that *can* remove the only metadata and
trigger this problem.  So to prepare for that, this CL fixes the
out-of-range problem.

Change-Id: Ie2d050d656512a7c725752427eaaec1c087f3c82
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206941
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/nnbd_migration/lib/src/edit_plan.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:convert';
+import 'dart:math';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/precedence.dart';
@@ -1424,8 +1425,8 @@ class _PassThroughBuilderImpl implements PassThroughBuilder {
       if (firstPlanIndex == 0 && lastPlanIndex == sequenceNodes!.length - 1) {
         // We're removing everything.  Try to remove additional whitespace so
         // that we're left with just `()`, `{}`, or `[]`.
-        var candidateFirstRemovalOffset =
-            planner._backAcrossWhitespace(firstRemovalOffset, node!.offset);
+        var candidateFirstRemovalOffset = planner._backAcrossWhitespace(
+            firstRemovalOffset, min(firstRemovalOffset, node!.offset));
         if (planner
             ._isJustAfter(candidateFirstRemovalOffset, const ['(', '[', '{'])) {
           var candidateLastRemovalEnd =

pkg/nnbd_migration/test/edit_plan_test.dart

@@ -888,6 +888,21 @@ class C {
     expect(changes.keys, [entry.offset]);
   }
 
+  Future<void>
+      test_remove_metadata_from_defaultFormalParameter_all_full_line() async {
+    await analyze('''
+f({
+  @deprecated
+  int? x}) {}''');
+    var deprecated = findNode.annotation('@deprecated');
+    checkPlan(
+        planner!.passThrough(deprecated.parent,
+            innerPlans: [planner!.removeNode(deprecated)]),
+        '''
+f({
+  int? x}) {}''');
+  }
+
   Future<void> test_remove_parameter() async {
     await analyze('f(int x, int y, int z) => null;');
     var parameter = findNode.simple('y').parent!;