Fix for the prefix/suffix diff computing.

pkg/analysis_server/test/services/correction/strings_test.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 test.services.correction.strings;
 
 import 'package:analysis_server/src/services/correction/strings.dart';
 import 'package:unittest/unittest.dart' hide isEmpty;
 
 import '../../reflective_tests.dart';
@@ skipping 17 matching lines @@
   }
 
   void test_compareStrings() {
     expect(compareStrings(null, null), 0);
     expect(compareStrings(null, 'b'), 1);
     expect(compareStrings('a', null), -1);
     expect(compareStrings('a', 'b'), -1);
     expect(compareStrings('b', 'a'), 1);
   }
 
+  void test_findCommonOverlap() {
+    expect(findCommonOverlap('', 'abcd'), 0);
+    expect(findCommonOverlap('abc', 'abcd'), 3);
+    expect(findCommonOverlap('123456', 'abcd'), 0);
+    expect(findCommonOverlap('123456xxx', 'xxxabcd'), 3);
+    expect(findCommonOverlap('123456', '56'), 2);
+    expect(findCommonOverlap('56', '56789'), 2);
+  }
+
+  void test_findCommonPrefix() {
+    expect(findCommonPrefix('abc', 'xyz'), 0);
+    expect(findCommonPrefix('1234abcdef', '1234xyz'), 4);
+    expect(findCommonPrefix('123', '123xyz'), 3);
+  }
+
+  void test_findCommonSuffix() {
+    expect(findCommonSuffix('abc', 'xyz'), 0);
+    expect(findCommonSuffix('abcdef1234', 'xyz1234'), 4);
+    expect(findCommonSuffix('123', 'xyz123'), 3);
+  }
+
   void test_isBlank() {
     expect(isBlank(null), isTrue);
     expect(isBlank(''), isTrue);
     expect(isBlank(' '), isTrue);
     expect(isBlank('\t'), isTrue);
     expect(isBlank('  '), isTrue);
     expect(isBlank('X'), isFalse);
   }
 
   void test_isDigit() {
@@ skipping 104 matching lines @@
     expect(repeat('abc', 3), 'abcabcabc');
   }
 
   void test_substringAfterLast() {
     expect(substringAfterLast('', '/'), '');
     expect(substringAfterLast('abc', ''), '');
     expect(substringAfterLast('abc', 'd'), 'abc');
     expect(substringAfterLast('abcbde', 'b'), 'de');
   }
 }