Fix NSME when file contents cannot be read (#260).

lib/src/formatter.dart

 // Copyright (c) 2015, 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 linter.src.formatter;
 
 import 'dart:io';
 import 'dart:math';
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/services/lint.dart';
 import 'package:linter/src/linter.dart';
 
 String getLineContents(int lineNumber, AnalysisError error) {
-  var path = error.source.fullName;
-  var file = new File(path);
-  if (file.existsSync()) {
+  String path = error.source.fullName;
+  File file = new File(path);
+  String failureDetails;
+  if (!file.existsSync()) {
+    failureDetails = 'file at $path does not exist';
+  } else {
     var lines = file.readAsLinesSync();
     var lineIndex = lineNumber - 1;
     if (lines.length > lineIndex) {
       return lines[lineIndex];
     }
+    failureDetails =
+        'line index ($lineIndex), outside of file line range (${lines.length})';
   }
-  return null;
+  throw new StateError('Unable to get contents for line: $failureDetails');
 }
 
 String pluralize(String word, int count) =>
     "$count ${count == 1 ? '$word' : '${word}s'}";
 
 String shorten(String fileRoot, String fullName) {
   if (fileRoot == null || !fullName.startsWith(fileRoot)) {
     return fullName;
   }
   return fullName.substring(fileRoot.length);
@@ skipping 232 matching lines @@
 
   void _writeLint(AnalysisError error, LineInfo lineInfo) {
     var offset = error.offset;
     var location = lineInfo.getLocation(offset);
     var line = location.lineNumber;
     var column = location.columnNumber;
 
     writeLint(error, offset: offset, column: column, line: line);
   }
 }