Fix example code just showing endless spinner when deployed from Windows (#14878)

* If example code is not found, render a message instead of an endless spinner
* Trim codeTags to avoid lookup issues with windows carriage returns
* Trim \r on Windows off the end of code blocks so they're consistently \n
* Add a test to ensure sample code parses if built on Windows
* Add a comment about why we trimRight()

Fixes #14820.

examples/flutter_gallery/lib/gallery/demo.dart

@@ -117,7 +117,7 @@ class FullScreenCodeDialogState extends State<FullScreenCodeDialog> {
     getExampleCode(widget.exampleCodeTag, DefaultAssetBundle.of(context)).then<Null>((String code) {
       if (mounted) {
         setState(() {
-          _exampleCode = code;
+          _exampleCode = code ?? 'Example code not found';
         });
       }
     });

examples/flutter_gallery/lib/gallery/example_code_parser.dart

@@ -33,7 +33,7 @@ Future<Null> _parseExampleCode(AssetBundle bundle) async {
       if (line.startsWith(_kStartTag)) {
         // Starting a new code block.
         codeBlock = <String>[];
-        codeTag = line.substring(_kStartTag.length);
+        codeTag = line.substring(_kStartTag.length).trim();
       } else {
         // Just skipping the line.
       }
@@ -46,7 +46,9 @@ Future<Null> _parseExampleCode(AssetBundle bundle) async {
         codeTag = null;
       } else {
         // Add to the current block
-        codeBlock.add(line);
+        // trimRight() to remove any \r on Windows
+        // without removing any useful indentation
+        codeBlock.add(line.trimRight());
       }
     }
   }

examples/flutter_gallery/test/example_code_parser_test.dart

@@ -18,6 +18,9 @@ void main() {
 
     final String codeSnippet1 = await getExampleCode('test_1', bundle);
     expect(codeSnippet1, 'test 1 0\ntest 1 1');
+
+    final String codeSnippet3 = await getExampleCode('test_2_windows_breaks', bundle);
+    expect(codeSnippet3, 'windows test 2 0\nwindows test 2 1');
   });
 }
 
@@ -32,6 +35,8 @@ test 0 1
 test 1 0
 test 1 1
 // END
+
+// START test_2_windows_breaks\r\nwindows test 2 0\r\nwindows test 2 1\r\n// END
 ''';
 
 class TestAssetBundle extends AssetBundle {