Fix LineInfo.getOffsetOfLineAfter().

Or we could remove it altogether.
It is not used outside of analyzer/analysis_server/analyzer_plugin.

R=brianwilkerson@google.com

Change-Id: I89c90fc0881f2b1602128f8aacdda2762c4e7920
Reviewed-on: https://dart-review.googlesource.com/54306
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/source/line_info.dart

@@ -119,6 +119,6 @@ class LineInfo {
    * containing the given [offset].
    */
   int getOffsetOfLineAfter(int offset) {
-    return getOffsetOfLine(getLocation(offset).lineNumber + 1);
+    return getOffsetOfLine(getLocation(offset).lineNumber);
   }
 }

pkg/analyzer/test/generated/utilities_test.dart

@@ -2546,26 +2546,43 @@ class LineInfoTest {
     }, throwsArgumentError);
   }
 
-  void test_firstLine() {
+  void test_getLocation_firstLine() {
     LineInfo info = new LineInfo(<int>[0, 12, 34]);
     CharacterLocation location = info.getLocation(4);
     expect(location.lineNumber, 1);
     expect(location.columnNumber, 5);
   }
 
-  void test_lastLine() {
+  void test_getLocation_lastLine() {
     LineInfo info = new LineInfo(<int>[0, 12, 34]);
     CharacterLocation location = info.getLocation(36);
     expect(location.lineNumber, 3);
     expect(location.columnNumber, 3);
   }
 
-  void test_middleLine() {
+  void test_getLocation_middleLine() {
     LineInfo info = new LineInfo(<int>[0, 12, 34]);
     CharacterLocation location = info.getLocation(12);
     expect(location.lineNumber, 2);
     expect(location.columnNumber, 1);
   }
+
+  void test_getOffsetOfLine() {
+    LineInfo info = new LineInfo(<int>[0, 12, 34]);
+    expect(0, info.getOffsetOfLine(0));
+    expect(12, info.getOffsetOfLine(1));
+    expect(34, info.getOffsetOfLine(2));
+  }
+
+  void test_getOffsetOfLineAfter() {
+    LineInfo info = new LineInfo(<int>[0, 12, 34]);
+
+    expect(info.getOffsetOfLineAfter(0), 12);
+    expect(info.getOffsetOfLineAfter(11), 12);
+
+    expect(info.getOffsetOfLineAfter(12), 34);
+    expect(info.getOffsetOfLineAfter(33), 34);
+  }
 }
 
 class ListGetter_NodeReplacerTest_test_adjacentStrings