Correct MaterialButton disabledColor (#30531)

packages/flutter/lib/src/material/button_theme.dart

@@ -491,7 +491,7 @@ class ButtonThemeData extends Diagnosticable {
     if (fillColor != null)
       return fillColor;
 
-    if (button is FlatButton || button is OutlineButton)
+    if (button is FlatButton || button is OutlineButton || button.runtimeType == MaterialButton)
       return null;
 
     if (button.enabled && button is RaisedButton && _buttonColor != null)

packages/flutter/lib/src/material/material_button.dart

@@ -263,7 +263,7 @@ class MaterialButton extends StatelessWidget {
     return RawMaterialButton(
       onPressed: onPressed,
       onHighlightChanged: onHighlightChanged,
-      fillColor: color,
+      fillColor: buttonTheme.getFillColor(this),
       textStyle: theme.textTheme.button.copyWith(color: buttonTheme.getTextColor(this)),
       highlightColor: highlightColor ?? theme.highlightColor,
       splashColor: splashColor ?? theme.splashColor,