Fix value computation for multi-line strings (issue 22993)

pkg/analyzer/test/generated/parser_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.parser_test;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/incremental_scanner.dart';
@@ skipping 8816 matching lines @@
         parse4("parseStringLiteral", "'a \${b} c \$this d'");
     NodeList<InterpolationElement> elements = literal.elements;
     expect(elements, hasLength(5));
     expect(elements[0] is InterpolationString, isTrue);
     expect(elements[1] is InterpolationExpression, isTrue);
     expect(elements[2] is InterpolationString, isTrue);
     expect(elements[3] is InterpolationExpression, isTrue);
     expect(elements[4] is InterpolationString, isTrue);
   }
 
+  void test_parseStringLiteral_multiline_trimmed() {
+    SimpleStringLiteral literal = parse4("parseStringLiteral", "''' \\\na'''");
+    expect(literal.literal, isNotNull);
+    expect(literal.value, "a");
+  }
+
+  void test_parseStringLiteral_multiline_untrimmed() {
+    SimpleStringLiteral literal = parse4("parseStringLiteral", "''' a\nb'''");
+    expect(literal.literal, isNotNull);
+    expect(literal.value, " a\nb");
+  }
+
   void test_parseStringLiteral_single() {
     SimpleStringLiteral literal = parse4("parseStringLiteral", "'a'");
     expect(literal.literal, isNotNull);
     expect(literal.value, "a");
   }
 
   void test_parseSuperConstructorInvocation_named() {
     SuperConstructorInvocation invocation =
         parse4("parseSuperConstructorInvocation", "super.a()");
     expect(invocation.argumentList, isNotNull);
@@ skipping 978 matching lines @@
         new Scanner(null, new CharSequenceReader(source), listener);
     Token tokenStream = scanner.tokenize();
     //
     // Parse the source.
     //
     Parser parser = new Parser(null, listener);
     return invokeParserMethodImpl(
         parser, methodName, <Object>[tokenStream], tokenStream) as Token;
   }
 }