improve fasta unterminated string recovery

pkg/compiler/lib/src/string_validator.dart

 // Copyright (c) 2012, 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.
 
 // Check the validity of string literals.
 
 library stringvalidator;
 
 import 'dart:collection';
 
 import 'common.dart';
 import 'package:front_end/src/fasta/scanner.dart' show Token;
 import 'tree/dartstring.dart' show DartString;
 import 'tree/nodes.dart' show StringQuoting;
 import 'package:front_end/src/fasta/scanner/characters.dart';
 
 class StringValidator {
   final DiagnosticReporter reporter;
 
   StringValidator(this.reporter);
 
   DartString validateInterpolationPart(Token token, StringQuoting quoting,
       {bool isFirst: false, bool isLast: false}) {
     String source = token.lexeme;
     int leftQuote = 0;
     int rightQuote = 0;
     if (isFirst) leftQuote = quoting.leftQuoteLength;
-    if (isLast) rightQuote = quoting.rightQuoteLength;
+    if (isLast) {
+      // Check for unterminated string
+      if (leftQuote < source.length &&
+          source[source.length - 1] == quoting.quoteChar) {
+        rightQuote = quoting.rightQuoteLength;

[ahe, 2017/05/29 15:05:30]

What happens with this string:

"""x"

+      }
+    }
     String content = copyWithoutQuotes(source, leftQuote, rightQuote);
     return validateString(
         token, token.charOffset + leftQuote, content, quoting);
   }
 
   static StringQuoting quotingFromString(String sourceString) {
     Iterator<int> source = sourceString.codeUnits.iterator;
     bool raw = false;
     int leftQuoteLength = 1;
     source.moveNext();
@@ skipping 166 matching lines @@
       }
     }
     // String literal successfully validated.
     if (quoting.raw || !containsEscape) {
       // A string without escapes could just as well have been raw.
       return new DartString.rawString(string, length);
     }
     return new DartString.escapedString(string, length);
   }
 }