Migration: turn off removeViaComments by default.

This was the original behavior, and it persists in some tests because
it's more trouble than it's worth to rewrite them.  But it's not the
behavior we want to ship to users.

Change-Id: Ibf1df628f62b01e934d05336bfe43a5a61268f93
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/147042
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/nnbd_migration/lib/src/fix_aggregator.dart

@@ -177,7 +177,7 @@ class FixAggregator extends UnifyingAstVisitor<void> {
   /// Runs the [FixAggregator] on a [unit] and returns the resulting edits.
   static Map<int, List<AtomicEdit>> run(
       CompilationUnit unit, String sourceText, Map<AstNode, NodeChange> changes,
-      {bool removeViaComments = true, bool warnOnWeakCode = false}) {
+      {bool removeViaComments = false, bool warnOnWeakCode = false}) {
     var planner = EditPlanner(unit.lineInfo, sourceText,
         removeViaComments: removeViaComments);
     var aggregator =

pkg/nnbd_migration/lib/src/nullability_migration_impl.dart

@@ -72,7 +72,7 @@ class NullabilityMigrationImpl implements NullabilityMigration {
   NullabilityMigrationImpl(NullabilityMigrationListener listener,
       {bool permissive: false,
       NullabilityMigrationInstrumentation instrumentation,
-      bool removeViaComments = true,
+      bool removeViaComments = false,
       bool warnOnWeakCode = true})
       : this._(listener, NullabilityGraph(instrumentation: instrumentation),
             permissive, instrumentation, removeViaComments, warnOnWeakCode);