Spelling fixes e to i.

pkg/front_end/test/fasta/parser/token_stream_rewriter_test.dart

 // Copyright (c) 2017, 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.
 
 import 'package:front_end/src/fasta/parser/token_stream_rewriter.dart';
 import 'package:front_end/src/fasta/scanner/token.dart';
 import 'package:front_end/src/scanner/token.dart'
     show BeginToken, Token, TokenType;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ skipping 115 matching lines @@
   }
 
   StringToken _makeToken(int charOffset, String text) {
     return new StringToken.fromString(null, text, charOffset);
   }
 }
 
 /// Concrete implementation of [TokenStreamRewriterTest] in which
 /// [Token.previous] values are set to null.
 ///
-/// This forces [TokenStreamRewriter] to use its more complex heursitc for
+/// This forces [TokenStreamRewriter] to use its more complex heuristic for
 /// finding previous tokens.
 @reflectiveTest
 class TokenStreamRewriterTest_NoPrevious extends TokenStreamRewriterTest {
   bool get setPrevious => false;
 }
 
 /// Concrete implementation of [TokenStreamRewriterTest] in which
 /// [Token.previous] values are set to non-null.
 ///
 /// Since [TokenStreamRewriter] makes use of [Token.previous] when it can,
 /// these tests do not exercise the more complex heuristics for finding previous
 /// tokens.
 @reflectiveTest
 class TokenStreamRewriterTest_UsingPrevious extends TokenStreamRewriterTest {
   bool get setPrevious => true;
 }