Fix source_span tests.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//712473002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/source_span@41614 260f80e4-7a28-3924-810f-c04153c831b5

pkgs/source_span/lib/src/file.dart

diff --git a/pkgs/source_span/lib/src/file.dart b/pkgs/source_span/lib/src/file.dart
index 5680733..14aa226 100644
--- a/pkgs/source_span/lib/src/file.dart
+++ b/pkgs/source_span/lib/src/file.dart
@@ -207,6 +207,11 @@
   FileSpan._(this.file, this._start, this._end) {
     if (_end < _start) {
       throw new ArgumentError('End $_end must come after start $_start.');
+    } else if (_end > file.length) {
+      throw new RangeError("End $_end must not be greater than the number "
+          "of characters in the file, ${file.length}.");
+    } else if (_start < 0) {
+      throw new RangeError("Start may not be negative, was $_start.");
     }
   }