Fix a blockquote issue (dart-lang/markdown#496)

Fix https://github.com/dart-lang/markdown/issues/495

pkgs/markdown/lib/src/block_syntaxes/blockquote_syntax.dart

diff --git a/pkgs/markdown/lib/src/block_syntaxes/blockquote_syntax.dart b/pkgs/markdown/lib/src/block_syntaxes/blockquote_syntax.dart
index 813cb2d..62f1903 100644
--- a/pkgs/markdown/lib/src/block_syntaxes/blockquote_syntax.dart
+++ b/pkgs/markdown/lib/src/block_syntaxes/blockquote_syntax.dart
@@ -33,8 +33,13 @@
         final markerStart = match.match.indexOf('>');
         int markerEnd;
         if (currentLine.length > 1) {
-          final nextChar = currentLine.codeUnitAt(markerStart + 1);
-          final hasSpace = nextChar == $tab || nextChar == $space;
+          var hasSpace = false;
+          // Check if there is a following space if the marker is not at the end
+          // of this line.
+          if (markerStart < currentLine.length - 1) {
+            final nextChar = currentLine.codeUnitAt(markerStart + 1);
+            hasSpace = nextChar == $tab || nextChar == $space;
+          }
           markerEnd = markerStart + (hasSpace ? 2 : 1);
         } else {
           markerEnd = markerStart + 1;

pkgs/markdown/test/original/block_quotes.unit

diff --git a/pkgs/markdown/test/original/block_quotes.unit b/pkgs/markdown/test/original/block_quotes.unit
index ac20218..548bc1a 100644
--- a/pkgs/markdown/test/original/block_quotes.unit
+++ b/pkgs/markdown/test/original/block_quotes.unit
@@ -47,3 +47,8 @@
 <p>quote</p>
 </blockquote>
 <hr />
+>>> issue #495
+  >
+<<<
+<blockquote>
+</blockquote>