Catch filesystem exception during output of dependencies delta.

When the frontend server attempts to compile dart code with an invalid
import (perhaps due to a missing entry in the .packages file), it will
get stuck in a bad state due to attempting to output said invalid import
uri as a new dependency.

Instead failures here should probably be caught and ignored, since the error messages are already reported through the existing diagnostics.

See also:

  - #36481

To reproduce this case:

1. Run the flutter gallery application (device unimportant).
2. Add an import for
`package:flutter_localizations/flutter_localizations.dart` in
`lib/main.dart`.
3. Request hot reload.

Before change:

```
Compiler message:
Error: Could not resolve the package 'flutter_localizations' in 'package:flutter_localizations/flutter_localizations.dart'.
lib/gallery/home.dart:12:8: Error: Not found: 'package:flutter_localizations/flutter_localizations.dart'
import 'package:flutter_localizations/flutter_localizations.dart';
       ^
Unhandled exception:
FileSystemException(uri=org-dartlang-untranslatable-uri:package%3Aflutter_localizations%2Fflutter_localizations.dart; message=StandardFileSystem only supports file:* and data:* URIs)
<asynchronous suspension>
<asynchronous suspension>
<asynchronous suspension>
<asynchronous suspension>
<asynchronous suspension>
```

It then gets stuck as from the perspective of the flutter tool the
reload request never completed.

After change:

```
Compiler message:
Error: Could not resolve the package 'flutter_localizations' in 'package:flutter_localizations/flutter_localizations.dart'.
lib/gallery/home.dart:12:8: Error: Not found: 'package:flutter_localizations/flutter_localizations.dart'
import 'package:flutter_localizations/flutter_localizations.dart';
       ^
Performing hot reload...
Reloaded 4 of 617 libraries in 837ms.

```

Change-Id: Ibb72624c492747a9339b15cb0b064edad8ebe4b8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/113135
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Zach Anderson <zra@google.com>

pkg/vm/lib/frontend_server.dart

@@ -441,11 +441,20 @@ class FrontendCompiler implements CompilerInterface {
       if (previouslyReportedDependencies.contains(uri)) {
         continue;
       }
-      _outputStream.writeln('+${await asFileUri(_fileSystem, uri)}');
+      try {
+        _outputStream.writeln('+${await asFileUri(_fileSystem, uri)}');
+      } on FileSystemException {
+        // Ignore errors from invalid import uris.
+      }
     }
     for (Uri uri in previouslyReportedDependencies) {
-      if (!uris.contains(uri)) {
+      if (uris.contains(uri)) {
+        continue;
+      }
+      try {
         _outputStream.writeln('-${await asFileUri(_fileSystem, uri)}');
+      } on FileSystemException {
+        // Ignore errors from invalid import uris.
       }
     }
     previouslyReportedDependencies = uris;