Remove deprecated Strings class.

samples/markdown/block_parser.dart

Index: samples/markdown/block_parser.dart
diff --git a/samples/markdown/block_parser.dart b/samples/markdown/block_parser.dart
index 16160cd18e2518f5b827895c12c3811a21bb4d1f..6bafc533cc66773d8563e9604a94b2acc9eecaf7 100644
--- a/samples/markdown/block_parser.dart
+++ b/samples/markdown/block_parser.dart
@@ -208,7 +208,7 @@ class CodeBlockSyntax extends BlockSyntax {
     childLines.add('');
 
     // Escape the code.
-    final escaped = escapeHtml(Strings.join(childLines, '\n'));
+    final escaped = escapeHtml(childLines.join('\n'));
 
     return new Element('pre', [new Element.text('code', escaped)]);
   }
@@ -249,7 +249,7 @@ class BlockHtmlSyntax extends BlockSyntax {
       parser.advance();
     }
 
-    return new Text(Strings.join(childLines, '\n'));
+    return new Text(childLines.join('\n'));
   }
 }
 
@@ -431,8 +431,7 @@ class ParagraphSyntax extends BlockSyntax {
       parser.advance();
     }
 
-    final contents = parser.document.parseInline(
-        Strings.join(childLines, '\n'));
+    final contents = parser.document.parseInline(childLines.join('\n'));
     return new Element('p', contents);
   }
 }