Update deprecated TextTheme TextStyle references (#222)

diff --git a/packages/flutter_markdown/lib/src/style_sheet.dart b/packages/flutter_markdown/lib/src/style_sheet.dart
index 5f66e0e..71c6294 100644
--- a/packages/flutter_markdown/lib/src/style_sheet.dart
+++ b/packages/flutter_markdown/lib/src/style_sheet.dart
@@ -76,34 +76,34 @@
 
   /// Creates a [MarkdownStyleSheet] from the [TextStyle]s in the provided [ThemeData].
   factory MarkdownStyleSheet.fromTheme(ThemeData theme) {
-    assert(theme?.textTheme?.body1?.fontSize != null);
+    assert(theme?.textTheme?.bodyText2?.fontSize != null);
     return MarkdownStyleSheet(
       a: const TextStyle(color: Colors.blue),
-      p: theme.textTheme.body1,
-      code: theme.textTheme.body1.copyWith(
+      p: theme.textTheme.bodyText2,
+      code: theme.textTheme.bodyText2.copyWith(
         backgroundColor: theme.cardTheme?.color ?? theme.cardColor,
         fontFamily: "monospace",
-        fontSize: theme.textTheme.body1.fontSize * 0.85,
+        fontSize: theme.textTheme.bodyText2.fontSize * 0.85,
       ),
-      h1: theme.textTheme.headline,
-      h2: theme.textTheme.title,
-      h3: theme.textTheme.subhead,
-      h4: theme.textTheme.body2,
-      h5: theme.textTheme.body2,
-      h6: theme.textTheme.body2,
+      h1: theme.textTheme.headline5,
+      h2: theme.textTheme.headline6,
+      h3: theme.textTheme.subtitle1,
+      h4: theme.textTheme.bodyText1,
+      h5: theme.textTheme.bodyText1,
+      h6: theme.textTheme.bodyText1,
       em: const TextStyle(fontStyle: FontStyle.italic),
       strong: const TextStyle(fontWeight: FontWeight.bold),
       del: const TextStyle(decoration: TextDecoration.lineThrough),
-      blockquote: theme.textTheme.body1,
-      img: theme.textTheme.body1,
-      checkbox: theme.textTheme.body1.copyWith(
+      blockquote: theme.textTheme.bodyText2,
+      img: theme.textTheme.bodyText2,
+      checkbox: theme.textTheme.bodyText2.copyWith(
         color: theme.primaryColor,
       ),
       blockSpacing: 8.0,
       listIndent: 24.0,
-      listBullet: theme.textTheme.body1,
+      listBullet: theme.textTheme.bodyText2,
       tableHead: const TextStyle(fontWeight: FontWeight.w600),
-      tableBody: theme.textTheme.body1,
+      tableBody: theme.textTheme.bodyText2,
       tableHeadAlign: TextAlign.center,
       tableBorder: TableBorder.all(
         color: theme.dividerColor,
@@ -243,31 +243,31 @@
   factory MarkdownStyleSheet.largeFromTheme(ThemeData theme) {
     return MarkdownStyleSheet(
       a: const TextStyle(color: Colors.blue),
-      p: theme.textTheme.body1,
-      code: theme.textTheme.body1.copyWith(
+      p: theme.textTheme.bodyText2,
+      code: theme.textTheme.bodyText2.copyWith(
         backgroundColor: theme.cardTheme?.color ?? theme.cardColor,
         fontFamily: "monospace",
-        fontSize: theme.textTheme.body1.fontSize * 0.85,
+        fontSize: theme.textTheme.bodyText2.fontSize * 0.85,
       ),
-      h1: theme.textTheme.display3,
-      h2: theme.textTheme.display2,
-      h3: theme.textTheme.display1,
-      h4: theme.textTheme.headline,
-      h5: theme.textTheme.title,
-      h6: theme.textTheme.subhead,
+      h1: theme.textTheme.headline2,
+      h2: theme.textTheme.headline3,
+      h3: theme.textTheme.headline4,
+      h4: theme.textTheme.headline5,
+      h5: theme.textTheme.headline6,
+      h6: theme.textTheme.subtitle1,
       em: const TextStyle(fontStyle: FontStyle.italic),
       strong: const TextStyle(fontWeight: FontWeight.bold),
       del: const TextStyle(decoration: TextDecoration.lineThrough),
-      blockquote: theme.textTheme.body1,
-      img: theme.textTheme.body1,
-      checkbox: theme.textTheme.body1.copyWith(
+      blockquote: theme.textTheme.bodyText2,
+      img: theme.textTheme.bodyText2,
+      checkbox: theme.textTheme.bodyText2.copyWith(
         color: theme.primaryColor,
       ),
       blockSpacing: 8.0,
       listIndent: 24.0,
-      listBullet: theme.textTheme.body1,
+      listBullet: theme.textTheme.bodyText2,
       tableHead: const TextStyle(fontWeight: FontWeight.w600),
-      tableBody: theme.textTheme.body1,
+      tableBody: theme.textTheme.bodyText2,
       tableHeadAlign: TextAlign.center,
       tableBorder: TableBorder.all(
         color: theme.dividerColor,
diff --git a/packages/flutter_markdown/test/flutter_markdown_test.dart b/packages/flutter_markdown/test/flutter_markdown_test.dart
index 055ef9a..bbc6597 100644
--- a/packages/flutter_markdown/test/flutter_markdown_test.dart
+++ b/packages/flutter_markdown/test/flutter_markdown_test.dart
@@ -15,9 +15,9 @@
 import 'package:mockito/mockito.dart';
 
 void main() {
-  TextTheme textTheme = Typography(platform: TargetPlatform.android)
+  final TextTheme textTheme = Typography.material2018(platform: TargetPlatform.android)
       .black
-      .merge(TextTheme(body1: TextStyle(fontSize: 12.0)));
+      .merge(TextTheme(bodyText2: TextStyle(fontSize: 12.0)));
 
   testWidgets('Simple string', (WidgetTester tester) async {
     await tester.pumpWidget(_boilerplate(const MarkdownBody(data: 'Hello')));
@@ -758,7 +758,7 @@
     testWidgets('MarkdownStyleSheet.fromTheme', (WidgetTester tester) async {
       final theme = ThemeData.dark().copyWith(
         textTheme: TextTheme(
-          body1: TextStyle(fontSize: 12.0),
+          bodyText2: TextStyle(fontSize: 12.0),
         ),
       );
 
@@ -768,59 +768,59 @@
       expect(style.a.color, Colors.blue);
 
       // p
-      expect(style.p, theme.textTheme.body1);
+      expect(style.p, theme.textTheme.bodyText2);
 
       // code
-      expect(style.code.color, theme.textTheme.body1.color);
-      expect(style.code.fontSize, theme.textTheme.body1.fontSize * 0.85);
+      expect(style.code.color, theme.textTheme.bodyText2.color);
+      expect(style.code.fontSize, theme.textTheme.bodyText2.fontSize * 0.85);
       expect(style.code.fontFamily, 'monospace');
       expect(style.code.backgroundColor, theme.cardColor);
 
       // H1
-      expect(style.h1, theme.textTheme.headline);
+      expect(style.h1, theme.textTheme.headline5);
 
       // H2
-      expect(style.h2, theme.textTheme.title);
+      expect(style.h2, theme.textTheme.headline6);
 
       // H3
-      expect(style.h3, theme.textTheme.subhead);
+      expect(style.h3, theme.textTheme.subtitle1);
 
       // H4
-      expect(style.h4, theme.textTheme.body2);
+      expect(style.h4, theme.textTheme.bodyText1);
 
       // H5
-      expect(style.h5, theme.textTheme.body2);
+      expect(style.h5, theme.textTheme.bodyText1);
 
       // H6
-      expect(style.h6, theme.textTheme.body2);
+      expect(style.h6, theme.textTheme.bodyText1);
 
       // em
       expect(style.em.fontStyle, FontStyle.italic);
-      expect(style.em.color, theme.textTheme.body1.color);
+      expect(style.em.color, theme.textTheme.bodyText2.color);
 
       // strong
       expect(style.strong.fontWeight, FontWeight.bold);
-      expect(style.strong.color, theme.textTheme.body1.color);
+      expect(style.strong.color, theme.textTheme.bodyText2.color);
 
       // del
       expect(style.del.decoration, TextDecoration.lineThrough);
-      expect(style.del.color, theme.textTheme.body1.color);
+      expect(style.del.color, theme.textTheme.bodyText2.color);
 
       // blockqoute
-      expect(style.blockquote, theme.textTheme.body1);
+      expect(style.blockquote, theme.textTheme.bodyText2);
 
       // img
-      expect(style.img, theme.textTheme.body1);
+      expect(style.img, theme.textTheme.bodyText2);
 
       // checkbox
       expect(style.checkbox.color, theme.primaryColor);
-      expect(style.checkbox.fontSize, theme.textTheme.body1.fontSize);
+      expect(style.checkbox.fontSize, theme.textTheme.bodyText2.fontSize);
 
       // tableHead
       expect(style.tableHead.fontWeight, FontWeight.w600);
 
       // tableBody
-      expect(style.tableBody, theme.textTheme.body1);
+      expect(style.tableBody, theme.textTheme.bodyText2);
     });
 
     testWidgets('merge', (WidgetTester tester) async {