Support multiple occurrences of text in context

test/span_test.dart

Index: test/span_test.dart
diff --git a/test/span_test.dart b/test/span_test.dart
index 39b0b9433ddc779998867a97ec8ebd60f77e7575..34d017446536b38d128fd8863ca562f012ea02a5 100644
--- a/test/span_test.dart
+++ b/test/span_test.dart
@@ -65,6 +65,21 @@ main() {
         new SourceSpanWithContext(start, end, "abc", "\n---abc--");
         new SourceSpanWithContext(start, end, "abc", "\n\n---abc--");
       });
+
+      test('text can occur multiple times in context', () {
+        var start1 = new SourceLocation(4, line: 55, column: 2);
+        var end1 = new SourceLocation(7, line: 55, column: 5);
+        var start2 = new SourceLocation(4, line: 55, column: 8);
+        var end2 = new SourceLocation(7, line: 55, column: 11);
+        new SourceSpanWithContext(start1, end1, "abc", "--abc---abc--\n");
+        new SourceSpanWithContext(start1, end1, "abc", "--abc--abc--\n");
+        new SourceSpanWithContext(start2, end2, "abc", "--abc---abc--\n");
+        new SourceSpanWithContext(start2, end2, "abc", "---abc--abc--\n");
+        expect(() => new SourceSpanWithContext(
+              start1, end1, "abc", "---abc--abc--\n"), throwsArgumentError);
+        expect(() => new SourceSpanWithContext(
+              start2, end2, "abc", "--abc--abc--\n"), throwsArgumentError);
+      });
     });
 
     group('for union()', () {