Fix incremental scanner's computation of scanEnd.

pkg/analyzer/test/generated/incremental_scanner_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 engine.incremental_scanner_test;
 
 import 'package:analyzer/src/generated/incremental_scanner.dart';
 import 'package:analyzer/src/generated/scanner.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 256 matching lines @@
   }
 
   void test_insert_splitIdentifier() {
     // "cob;"
     // "cow.b;"
     _scan("co", "", "w.", "b;");
     _assertTokens(-1, 3, ["cow", ".", "b", ";"]);
     expect(_incrementalScanner.hasNonWhitespaceChange, isTrue);
   }
 
+  void test_insert_tokens_within_whitespace() {
+    // "a      ;"
+    // "a   +b   ;"
+    _scan("a   ", "", "+b", "   ;");
+    _assertTokens(0, 3, ["a", "+", "b", ";"]);
+    expect(_incrementalScanner.hasNonWhitespaceChange, isTrue);
+  }
+
   void test_insert_whitespace_beginning_beforeToken() {
     // "a + b;"
     // " a + b;"
     _scan("", "", " ", "a + b;");
     _assertTokens(0, 1, ["a", "+", "b", ";"]);
     expect(_incrementalScanner.hasNonWhitespaceChange, isFalse);
   }
 
   void test_insert_whitespace_betweenTokens() {
     // "a + b;"
@@ skipping 262 matching lines @@
         incrementalToken.type,
         same(TokenType.EOF),
         reason: "Too many tokens");
     expect(
         modifiedTokens.type,
         same(TokenType.EOF),
         reason: "Not enough tokens");
     // TODO(brianwilkerson) Verify that the errors are correct?
   }
 }