text: Bundle 'Source Code Pro' font into the app

We'll probably want to bundle more fonts into the app, like Source
Sans 3, which we use for most text in Zulip web.

But now at least we can see the process works! :)

Fixes: #64

Author: chrisbobbe

assets/SourceCodePro-Italic-VariableFont_wght.ttf

@@ -355,10 +355,11 @@ InlineSpan inlineCode(InlineCodeNode node) {
 
   // Use a light gray background, instead of a border.
   return TextSpan(
-      style: kMonospaceTextStyle.merge(const TextStyle(
-        backgroundColor: Color(0xffeeeeee),
-        fontSize: 0.825 * kBaseFontSize,
-      )),
+      style: kMonospaceTextStyle
+        .merge(const TextStyle(
+          backgroundColor: Color(0xffeeeeee),
+          fontSize: 0.825 * kBaseFontSize))
+        .merge(weightVariableTextStyle(null)), // TODO(a11y) pass a BuildContext
       children: _buildInlineList(node.nodes));
 
   // Another fun solution -- we can in fact have a border!  Like so:
@@ -378,10 +379,11 @@ InlineSpan inlineCode(InlineCodeNode node) {
   // ]);
 }
 
-final _kCodeStyle = kMonospaceTextStyle.merge(const TextStyle(
-  backgroundColor: Color.fromRGBO(255, 255, 255, 1),
-  fontSize: 0.825 * kBaseFontSize,
-));
+final _kCodeStyle = kMonospaceTextStyle
+  .merge(const TextStyle(
+    backgroundColor: Color.fromRGBO(255, 255, 255, 1),
+    fontSize: 0.825 * kBaseFontSize))
+  .merge(weightVariableTextStyle(null)); // TODO(a11y) pass a BuildContext
 
 // const _kInlineCodeLeftBracket = '⸤';
 // const _kInlineCodeRightBracket = '⸣';
@@ -638,4 +640,6 @@ InlineSpan _errorUnimplemented(UnimplementedNode node) {
 
 const errorStyle = TextStyle(fontWeight: FontWeight.bold, color: Colors.red);
 
-final errorCodeStyle = kMonospaceTextStyle.merge(const TextStyle(color: Colors.red));
+final errorCodeStyle = kMonospaceTextStyle
+  .merge(const TextStyle(color: Colors.red))
+  .merge(weightVariableTextStyle(null)); // TODO(a11y) pass a BuildContext

assets/SourceCodePro-VariableFont_wght.ttf

@@ -4,13 +4,18 @@ import 'package:flutter/widgets.dart';
 
 /// A mergeable [TextStyle] with 'Source Code Pro' and platform-aware fallbacks.
 ///
+/// Callers should also call [weightVariableTextStyle] and merge that in too,
+/// because for this font, we use "variable font" assets with a "wght" axis.
+///
 /// Example:
 ///
 /// ```dart
-/// kMonospaceTextStyle.merge(const TextStyle(color: Colors.red))
+/// kMonospaceTextStyle
+///   .merge(const TextStyle(color: Colors.red))
+///   .merge(weightVariableTextStyle(context))
 /// ```
 final TextStyle kMonospaceTextStyle = TextStyle(
-  fontFamily: 'Source Code Pro', // TODO supply font
+  fontFamily: 'Source Code Pro',
 
   // Oddly, iOS doesn't handle 'monospace':
   //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20monospace.20font.20fallback/near/1570622

lib/widgets/content.dart

@@ -94,22 +94,9 @@ flutter:
   # For details regarding adding assets from package dependencies, see
   # https://flutter.dev/assets-and-images/#from-packages
 
-  # To add custom fonts to your application, add a fonts section here,
-  # in this "flutter" section. Each entry in this list should have a
-  # "family" key with the font family name, and a "fonts" key with a
-  # list giving the asset and other descriptors for the font. For
-  # example:
-  # fonts:
-  #   - family: Schyler
-  #     fonts:
-  #       - asset: fonts/Schyler-Regular.ttf
-  #       - asset: fonts/Schyler-Italic.ttf
-  #         style: italic
-  #   - family: Trajan Pro
-  #     fonts:
-  #       - asset: fonts/TrajanPro.ttf
-  #       - asset: fonts/TrajanPro_Bold.ttf
-  #         weight: 700
-  #
-  # For details regarding fonts from package dependencies,
-  # see https://flutter.dev/custom-fonts/#from-packages
+  fonts:
+    - family: Source Code Pro
+      fonts:
+        - asset: assets/SourceCodePro-VariableFont_wght.ttf
+        - asset: assets/SourceCodePro-Italic-VariableFont_wght.ttf
+          style: italic