[messages] Add LocatableDiagnostic.atSourceRange.

This expands the analyzer's literate API for diagnostic reporting so
that diagnostic locations can be specified using a `SourceRange`
object.

Change-Id: I6a6a696470fd1ef2682e88e6c4dc7b467ed3e39b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/467643
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/base/errors.dart

@@ -10,6 +10,7 @@ import 'dart:math';
 import 'package:_fe_analyzer_shared/src/base/analyzer_public_api.dart';
 import 'package:_fe_analyzer_shared/src/base/diagnostic_message.dart';
 import 'package:_fe_analyzer_shared/src/base/source.dart';
+import 'package:_fe_analyzer_shared/src/base/source_range.dart';
 import 'package:_fe_analyzer_shared/src/base/syntactic_entity.dart';
 import 'package:source_span/source_span.dart';
 
@@ -571,6 +572,10 @@ base mixin DiagnosticWithoutArguments on DiagnosticCodeImpl
   LocatedDiagnostic atOffset({required int offset, required int length}) =>
       new LocatedDiagnostic(this, offset, length);
 
+  @override
+  LocatedDiagnostic atSourceRange(SourceRange sourceRange) =>
+      atOffset(offset: sourceRange.offset, length: sourceRange.length);
+
   @override
   LocatedDiagnostic atSourceSpan(SourceSpan span) =>
       atOffset(offset: span.start.offset, length: span.length);
@@ -633,6 +638,12 @@ abstract final class LocatableDiagnostic {
   /// The result may be passed to [DiagnosticReporter.reportError].
   LocatedDiagnostic atOffset({required int offset, required int length});
 
+  /// Converts this diagnostic to a [LocatedDiagnostic] by applying it to a
+  /// location in the source code.
+  ///
+  /// The result may be passed to [DiagnosticReporter.reportError].
+  LocatedDiagnostic atSourceRange(SourceRange sourceRange);
+
   /// Converts this diagnostic to a [LocatedDiagnostic] by applying it to a
   /// location in the source code.
   ///
@@ -683,6 +694,10 @@ final class LocatableDiagnosticImpl implements LocatableDiagnostic {
   LocatedDiagnostic atOffset({required int offset, required int length}) =>
       new LocatedDiagnostic(this, offset, length);
 
+  @override
+  LocatedDiagnostic atSourceRange(SourceRange sourceRange) =>
+      atOffset(offset: sourceRange.offset, length: sourceRange.length);
+
   @override
   LocatedDiagnostic atSourceSpan(SourceSpan span) =>
       atOffset(offset: span.start.offset, length: span.length);