Add fontFamily to ThemeData constructor (#9016)

Otherwise it's somewhat verbose to configure all the text themes.

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

@@ -93,6 +93,7 @@ class ThemeData {
     Color indicatorColor,
     Color hintColor,
     Color errorColor,
+    String fontFamily,
     TextTheme textTheme,
     TextTheme primaryTextTheme,
     TextTheme accentTextTheme,
@@ -137,6 +138,11 @@ class ThemeData {
     textTheme ??= isDark ? typography.white : typography.black;
     primaryTextTheme ??= primaryIsDark ? typography.white : typography.black;
     accentTextTheme ??= accentIsDark ? typography.white : typography.black;
+    if (fontFamily != null) {
+      textTheme = textTheme.apply(fontFamily: fontFamily);
+      primaryTextTheme = primaryTextTheme.apply(fontFamily: fontFamily);
+      accentTextTheme = accentTextTheme.apply(fontFamily: fontFamily);
+    }
     return new ThemeData.raw(
       brightness: brightness,
       primaryColor: primaryColor,

packages/flutter/test/material/theme_data_test.dart

@@ -81,4 +81,12 @@ void main() {
     expect(lightTheme.accentTextTheme.title.color, typography.black.title.color);
     expect(darkTheme.accentTextTheme.title.color, typography.white.title.color);
   });
+
+  test('Can control fontFamily', () {
+    final ThemeData themeData = new ThemeData(fontFamily: 'Ahem');
+
+    expect(themeData.textTheme.body2.fontFamily, equals('Ahem'));
+    expect(themeData.primaryTextTheme.title.fontFamily, equals('Ahem'));
+    expect(themeData.accentTextTheme.display4.fontFamily, equals('Ahem'));
+  });
 }