Fix incremental scanner's computation of scanEnd.

pkg/analyzer/test/generated/incremental_scanner_test.dart

Index: pkg/analyzer/test/generated/incremental_scanner_test.dart
diff --git a/pkg/analyzer/test/generated/incremental_scanner_test.dart b/pkg/analyzer/test/generated/incremental_scanner_test.dart
index f9889f3c02b07e2246f80b19127b7607aca28292..37cd8005609177736046fec18490eefb21b11205 100644
--- a/pkg/analyzer/test/generated/incremental_scanner_test.dart
+++ b/pkg/analyzer/test/generated/incremental_scanner_test.dart
@@ -274,6 +274,14 @@ class IncrementalScannerTest extends EngineTestCase {
     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;"