Repository: flutter
Issue: [material] allow all overrides Theme().copyWith() 
Labels: c: new feature,framework,f: material design,c: proposal,P3,team-design,triaged-design

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## Steps to Reproduce

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While creating a copy of theme primarySwatch and fontFamily isn't available for override.
Is this normal or a limitation of some kind?

<img width="789" alt="Screenshot 2019-11-18 at 1 41 13 PM" src="https://user-images.githubusercontent.com/13887407/69035491-d1db1200-0a09-11ea-9da8-b32060423d94.png">

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**Target Platform:** MacOS
**Target OS version/browser:** MacOS
**Devices:** MacOSS


## Logs

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```
[✓] Flutter (Channel master, v1.12.3-pre.45, on Mac OS X 10.14.6 18G95, locale en-GB)
    • Flutter version 1.12.3-pre.45 at /Users/ayushpgupta/development/flutter_master/flutter
    • Framework revision c82c587b33 (33 hours ago), 2019-11-16 18:02:51 -0500
    • Engine revision 7ef587220a
    • Dart version 2.7.0

 
[!] Android toolchain - develop for Android devices (Android SDK version 29.0.2)
    • Android SDK at /Users/ayushpgupta/Library/Android/sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-29, build-tools 29.0.2
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)
    ! Some Android licenses not accepted.  To resolve this, run: flutter doctor --android-licenses

[✓] Xcode - develop for iOS and macOS (Xcode 10.3)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 10.3, Build version 10G8
    • CocoaPods version 1.6.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 3.5)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 39.0.3
    • Dart plugin version 191.8423
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] IntelliJ IDEA Ultimate Edition (version 2019.2.3)
    • IntelliJ at /Applications/IntelliJ IDEA.app
    • Flutter plugin version 41.2.3
    • Dart plugin version 192.7402

[✓] Connected device (3 available)
    • macOS      • macOS      • darwin-x64     • Mac OS X 10.14.6 18G95
    • Chrome     • chrome     • web-javascript • Google Chrome 78.0.3904.97
    • Web Server • web-server • web-javascript • Flutter Tools

! Doctor found issues in 1 category.

```