Fix source_span tests.

pkg/source_span/lib/src/file.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library source_span.file;
 
 import 'dart:math' as math;
 import 'dart:typed_data';
 
 import 'package:path/path.dart' as p;
@@ skipping 189 matching lines @@
   final int _end;
 
   FileLocation get start => new FileLocation._(file, _start);
   FileLocation get end => new FileLocation._(file, _end);
 
   String get text => file.getText(start.offset, end.offset);
 
   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.");
     }
   }
 
   SourceSpan union(SourceSpan other) {
     if (other is! FileSpan) return super.union(other);
 
     var span = expand(other);
     var beginSpan = span.start == this.start ? this : other;
     var endSpan = span.end == this.end ? this : other;
 
@@ skipping 49 matching lines @@
     }
     if (!textLine.endsWith('\n')) buffer.write('\n');
 
     buffer.write(' ' * column);
     if (color != null) buffer.write(color);
     buffer.write('^' * math.max(toColumn - column, 1));
     if (color != null) buffer.write(colors.NONE);
     return buffer.toString();
   }
 }