title: Resource IDs in arb files must Start with capital ?(flutter localization: i18n and l10n support)
body: <p>I have a flutter project, and updated it to nullsafety just now. And I found that intl_translation plugin are not support for nullsafety. So I remove it and want to use the intl comes with the framwork of flutter.</p>
  <p>According to the <a href="https://flutter.dev/docs/development/accessibility-and-localization/internationalization" rel="nofollow noreferrer">official documents</a>, I add 2 lines: <code>intl: ^0.17.0 # </code> , <code>generate: true</code> and new a file named <code>l10n.yaml</code> .</p>
  <p>l10n.yaml 's content as follow:</p>
  <pre><code>arb-dir: lib/l10n
  template-arb-file: intl_en.arb
  output-localization-file: lib/generated/intl/l10n.dart
  </code></pre>
  <p>Then my file tree as follow:</p>
  <pre><code>lib
  ├── data
  │   ├── ......
  ├── generated
  │   ├── intl
  │   │   ├── messages_all.dart
  │   │   ├── messages_en.dart
  │   │   └── messages_zh.dart
  │   └── l10n.dart
  ├── l10n
  │   ├── intl_en.arb
  │   └── intl_zh.arb
  ├── ......
  ├── ......
  ├── ui
  │   ├── ......
  └── utilities.dart
  </code></pre>
  <p>Then I <code>flutter run </code>my project. The ERRORS show that</p>
  <pre><code>Invalid ARB resource name &quot;UPCredit&quot; in LocalFile: '.../lib/l10n/intl_en.arb'.
  Resources names must be valid Dart method names: they have to be camel case, cannot start with a number or underscore, and cannot contain non-alphanumeric characters.
  Generating synthetic localizations package has failed.
  </code></pre>
  <p>The error position is in my <code>intl_en.arb</code> as follows:</p>
  <pre><code>&quot;UPxxt&quot;: &quot;Uxxx Cxxxit&quot;,
  &quot;UPxit&quot;: &quot;Unxxx Dxxxt&quot;,
  &quot;UPSxit&quot;: &quot;Uxxx Sxxxed xxxit&quot;,
  </code></pre>
  <p><strong>MY PROBLEM</strong>:
  NoBody talk me not to do like this(Maybe I can not find something about it). Can anybody help me about how to solve it? Something about document(how to find this?).</p>