[Icons] Updating default icon theme values. (#20840)

Closes #3188

examples/stocks/test/icon_color_test.dart

@@ -74,7 +74,7 @@ void main() {
 
     // check the colour of the icon - light mode
     checkIconColor(tester, 'Stock List', Colors.purple); // theme primary color
-    checkIconColor(tester, 'Account Balance', Colors.black26); // disabled
+    checkIconColor(tester, 'Account Balance', Colors.black38); // disabled
     checkIconColor(tester, 'About', Colors.black45); // enabled
 
     // switch to dark mode

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

@@ -411,11 +411,11 @@ class MaterialButton extends StatelessWidget {
       case ButtonTextTheme.normal:
         return enabled
           ? (themeIsDark ? Colors.white : Colors.black87)
-          : (themeIsDark ? Colors.white30 : Colors.black26);
+          : theme.disabledColor;
       case ButtonTextTheme.accent:
         return enabled
           ? theme.accentColor
-          : (themeIsDark ? Colors.white30 : Colors.black26);
+          : theme.disabledColor;
       case ButtonTextTheme.primary:
         return enabled
           ? (fillIsDark ? Colors.white : Colors.black)

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

@@ -167,7 +167,7 @@ class ThemeData extends Diagnosticable {
     splashFactory ??= InkSplash.splashFactory;
     selectedRowColor ??= Colors.grey[100];
     unselectedWidgetColor ??= isDark ? Colors.white70 : Colors.black54;
-    disabledColor ??= isDark ? Colors.white30 : Colors.black26;
+    disabledColor ??= isDark ? Colors.white30 : Colors.black38;
     buttonColor ??= isDark ? primarySwatch[600] : Colors.grey[300];
     buttonTheme ??= const ButtonThemeData();
     // Spec doesn't specify a dark theme secondaryHeaderColor, this is a guess.
@@ -182,9 +182,9 @@ class ThemeData extends Diagnosticable {
     hintColor ??= isDark ?  const Color(0x80FFFFFF) : const Color(0x8A000000);
     errorColor ??= Colors.red[700];
     inputDecorationTheme ??= const InputDecorationTheme();
-    iconTheme ??= isDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black);
     primaryIconTheme ??= primaryIsDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black);
     accentIconTheme ??= accentIsDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black);
+    iconTheme ??= isDark ? const IconThemeData(color: Colors.white) : const IconThemeData(color: Colors.black87);
     platform ??= defaultTargetPlatform;
     final Typography typography = new Typography(platform: platform);
     final TextTheme defaultTextTheme = isDark ? typography.white : typography.black;

packages/flutter/test/material/buttons_test.dart

@@ -310,7 +310,27 @@ void main() {
     );
   });
 
-  testWidgets('Disabled MaterialButton has same semantic size as enabled and exposes disabled semantics', (WidgetTester tester) async {
+  testWidgets('Disabled MaterialButton has correct default text color', (WidgetTester tester) async {
+    const String testText = 'Disabled';
+    const Widget buttonWidget = Directionality(
+      textDirection: TextDirection.ltr,
+      child: Material(
+        child: Center(
+          child: MaterialButton(
+            onPressed: null,
+            child: Text(testText), // button is disabled
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpWidget(buttonWidget);
+
+    final RichText text = tester.widget<RichText>(find.byType(RichText));
+    expect(text.text.style.color, Colors.black38);
+  });
+
+    testWidgets('Disabled MaterialButton has same semantic size as enabled and exposes disabled semantics', (WidgetTester tester) async {
     final SemanticsTester semantics = new SemanticsTester(tester);
 
     final Rect expectedButtonSize = new Rect.fromLTRB(0.0, 0.0, 116.0, 48.0);

packages/flutter/test/material/expand_icon_test.dart

@@ -37,7 +37,7 @@ void main() {
     );
 
     final IconTheme iconTheme = tester.firstWidget(find.byType(IconTheme).last);
-    expect(iconTheme.data.color, equals(Colors.black26));
+    expect(iconTheme.data.color, equals(Colors.black38));
   });
 
   testWidgets('ExpandIcon test isExpanded does not trigger callback', (WidgetTester tester) async {