Repository: flutter
Issue: ExpansionTile subtitle should have the same color as ListTile subtitle
Labels: framework,f: material design,c: API break,has reproducible steps,found in release: 3.0,found in release: 3.1,team-design,triaged-design

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## Steps to Reproduce

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Add ExpansionTile and ListTile with the exact same contents of title and subtitle in one list and compare

**Expected results:** <!-- what did you want to see? -->
There should be no difference in both ExpansionTile and ListTile.

**Actual results:** <!-- what did you see? -->
As you can see, there is a color difference between both of them.
<img src="https://user-images.githubusercontent.com/5113916/80529947-94a38480-8998-11ea-8683-5e5c600117ef.png" width="420">

<details>
  <summary>Logs</summary>

<!--
     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
-->

```
$ flutter analyze
Running "flutter pub get" in stepslow...                            2,0s
Analyzing stepslow...                                                   

warning • 'avoid_escaping_inner_quotes' is not a recognized lint rule •
       analysis_options.yaml:18:7 • undefined_lint_warning
warning • 'avoid_redundant_argument_values' is not a recognized lint rule •
       analysis_options.yaml:28:7 • undefined_lint_warning
warning • 'leading_newlines_in_multiline_strings' is not a recognized lint rule
       • analysis_options.yaml:68:7 • undefined_lint_warning
warning • 'missing_whitespace_between_adjacent_strings' is not a recognized lint
       rule • analysis_options.yaml:74:7 • undefined_lint_warning
warning • 'no_logic_in_create_state' is not a recognized lint rule •
       analysis_options.yaml:77:7 • undefined_lint_warning
warning • 'no_runtimeType_toString' is not a recognized lint rule •
       analysis_options.yaml:78:7 • undefined_lint_warning
warning • 'unnecessary_raw_strings' is not a recognized lint rule •
       analysis_options.yaml:151:7 • undefined_lint_warning
warning • 'unnecessary_string_escapes' is not a recognized lint rule •
       analysis_options.yaml:153:7 • undefined_lint_warning
warning • 'unnecessary_string_interpolations' is not a recognized lint rule •
       analysis_options.yaml:154:7 • undefined_lint_warning
warning • 'use_key_in_widget_constructors' is not a recognized lint rule •
       analysis_options.yaml:160:7 • undefined_lint_warning
warning • 'use_raw_strings' is not a recognized lint rule •
       analysis_options.yaml:161:7 • undefined_lint_warning

11 issues found. (ran in 14.7s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->

```
$ flutter doctor -v
[✓] Flutter (Channel stable, v1.12.13+hotfix.9, on Linux, locale cs_CZ.UTF-8)
    • Flutter version 1.12.13+hotfix.9 at /home/pavel/flutter
    • Framework revision f139b11009 (4 weeks ago), 2020-03-30 13:57:30 -0700
    • Engine revision af51afceb8
    • Dart version 2.7.2

 
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
    • Android SDK at /home/pavel/Android/Sdk
    • Android NDK location not configured (optional; useful for native profiling
      support)
    • Platform android-29, build-tools 29.0.3
    • Java binary at: /opt/android-studio/jre/bin/java
    • Java version OpenJDK Runtime Environment (build
      1.8.0_212-release-1586-b4-5784211)
    • All Android licenses accepted.

[!] Android Studio (version 3.6)
    • Android Studio at /opt/android-studio
    ✗ Flutter plugin not installed; this adds Flutter specific functionality.
    ✗ Dart plugin not installed; this adds Dart specific functionality.
    • Java version OpenJDK Runtime Environment (build
      1.8.0_212-release-1586-b4-5784211)

[✓] Connected device (1 available)
    • HUAWEI CAM L21 • LHTDU16A22004756 • android-arm64 • Android 6.0 (API 23)

! Doctor found issues in 1 category.
```

Flutter 1.17 also affected
</details>