improve fasta unterminated string recovery

pkg/front_end/lib/src/scanner/token.dart

Index: pkg/front_end/lib/src/scanner/token.dart
diff --git a/pkg/front_end/lib/src/scanner/token.dart b/pkg/front_end/lib/src/scanner/token.dart
index c4ce6f1e55239614c0f56350ba96ea2f8e7fc2b3..c208c134da19df03cbc4634aaa7f14a5b1a36bef 100644
--- a/pkg/front_end/lib/src/scanner/token.dart
+++ b/pkg/front_end/lib/src/scanner/token.dart
@@ -716,17 +716,23 @@ class SyntheticKeywordToken extends KeywordToken {
  * A token whose value is independent of it's type.
  */
 class SyntheticStringToken extends StringToken {
+  final int _length;
+
   /**
    * Initialize a newly created token to represent a token of the given [type]
-   * with the given [value] at the given [offset].
+   * with the given [value] at the given [offset]. If the [length] is
+   * not specified, then it defaults to the length of [value].
    */
-  SyntheticStringToken(TokenType type, String value, int offset)
+  SyntheticStringToken(TokenType type, String value, int offset, [this._length])
       : super(type, value, offset);
 
   @override
   bool get isSynthetic => true;
 
   @override
+  int get length => _length ?? super.length;
+
+  @override
   Token copy() => new SyntheticStringToken(type, _value, offset);
 }