Analyzer: fix 'ignore:' comments at the end of a line.

'//ignore:' comments that sit next to code should not invalidate errors on the
next line.

Fixes https://github.com/dart-lang/sdk/issues/28467

Bug: https://github.com/dart-lang/sdk/issues/28467
Change-Id: I15b1787e5fe4dadcdc72c18409e0b82df11060af
Reviewed-on: https://dart-review.googlesource.com/29548
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart

@@ -462,9 +462,7 @@ class LibraryAnalyzer {
     bool isIgnored(AnalysisError error) {
       int errorLine = lineInfo.getLocation(error.offset).lineNumber;
       String errorCode = error.errorCode.name.toLowerCase();
-      // Ignores can be on the line or just preceding the error.
-      return ignoreInfo.ignoredAt(errorCode, errorLine) ||
-          ignoreInfo.ignoredAt(errorCode, errorLine - 1);
+      return ignoreInfo.ignoredAt(errorCode, errorLine);
     }
 
     return errors.where((AnalysisError e) => !isIgnored(e)).toList();

pkg/analyzer/lib/src/task/dart.dart

@@ -2575,9 +2575,7 @@ class DartErrorsTask extends SourceBasedAnalysisTask {
     bool isIgnored(AnalysisError error) {
       int errorLine = lineInfo.getLocation(error.offset).lineNumber;
       String errorCode = error.errorCode.name.toLowerCase();
-      // Ignores can be on the line or just preceding the error.
-      return ignoreInfo.ignoredAt(errorCode, errorLine) ||
-          ignoreInfo.ignoredAt(errorCode, errorLine - 1);
+      return ignoreInfo.ignoredAt(errorCode, errorLine);
     }
 
     return errors.where((AnalysisError e) => !isIgnored(e)).toList();
@@ -3007,7 +3005,7 @@ class IgnoreInfo {
    * Resulting codes may be in a list ('error_code_1,error_code2').
    */
   static final RegExp _IGNORE_MATCHER =
-      new RegExp(r'//[ ]*ignore:(.*)$', multiLine: true);
+      new RegExp(r'//+[ ]*ignore:(.*)$', multiLine: true);
 
   /**
    * A regular expression for matching 'ignore_for_file' comments.  Produces
@@ -3084,7 +3082,19 @@ class IgnoreInfo {
           .group(1)
           .split(',')
           .map((String code) => code.trim().toLowerCase());
-      ignoreInfo.addAll(info.getLocation(match.start).lineNumber, codes);
+      LineInfo_Location location = info.getLocation(match.start);
+      int lineNumber = location.lineNumber;
+      String beforeMatch = content.substring(
+          info.getOffsetOfLine(lineNumber - 1),
+          info.getOffsetOfLine(lineNumber - 1) + location.columnNumber - 1);
+
+      if (beforeMatch.trim().isEmpty) {
+        // The comment is on its own line, so it refers to the next line.
+        ignoreInfo.addAll(lineNumber + 1, codes);
+      } else {
+        // The comment sits next to code, so it refers to its own line.
+        ignoreInfo.addAll(lineNumber, codes);
+      }
     }
     for (Match match in fileMatches) {
       Iterable<String> codes = match

pkg/analyzer/test/generated/error_suppression_kernel_test.dart

@@ -134,4 +134,11 @@ class ErrorSuppressionTest_Kernel extends ErrorSuppressionTest_Driver {
     //          1 errors of type CompileTimeErrorCode.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE, found 0
     await super.test_no_ignores();
   }
+
+  @override
+  @failingTest
+  test_trailing_not_above() async {
+    // Expected 1 errors of type StaticTypeWarningCode.INVALID_ASSIGNMENT, found 0
+    await super.test_trailing_not_above();
+  }
 }

pkg/analyzer/test/generated/error_suppression_test.dart

@@ -215,6 +215,17 @@ const String y = x; //INVALID_ASSIGNMENT, CONST_INITIALIZED_WITH_NON_CONSTANT_VA
     assertErrors(source, []);
   }
 
+  test_trailing_not_above() async {
+    Source source = addSource('''
+int x = ''; // ignore: invalid_assignment
+int y = '';
+''');
+    await computeAnalysisResult(source);
+    assertErrors(source, [
+      StaticTypeWarningCode.INVALID_ASSIGNMENT,
+    ]);
+  }
+
   test_no_ignores() async {
     Source source = addSource('''
 int x = '';  //INVALID_ASSIGNMENT

pkg/analyzer/test/src/task/dart_test.dart

@@ -4098,7 +4098,7 @@ bar(); //ignore: error_code, error_code_2
 
     IgnoreInfo info = outputs[IGNORE_INFO];
     expect(info.ignores.keys, hasLength(3));
-    expect(info.ignores[1].first, 'error_code');
+    expect(info.ignores[2].first, 'error_code');
     expect(info.ignores[4].first, 'error_code_2');
     expect(info.ignores[5], unorderedEquals(['error_code', 'error_code_2']));
     expect(info.ignoreForFiles,