improve fasta unterminated string recovery

pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart

Index: pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
index d4a97e0f255a561ece55c8425760c39f873f68e4..2d072322b1b53eff4873c69bbc5c44a99bdf1a36 100644
--- a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
@@ -8,7 +8,8 @@ import 'dart:collection' show ListMixin;
 
 import 'dart:typed_data' show Uint16List, Uint32List;
 
-import '../../scanner/token.dart' show BeginToken, Token, TokenType;
+import '../../scanner/token.dart'
+    show BeginToken, SyntheticStringToken, Token, TokenType;
 
 import '../scanner.dart'
     show ErrorToken, Keyword, Scanner, buildUnexpectedCharacterToken;
@@ -1024,7 +1025,7 @@ abstract class AbstractScanner implements Scanner {
               identical(next, $CR) ||
               identical(next, $EOF))) {
         if (!asciiOnly) handleUnicode(start);
-        return unterminatedString(quoteChar);
+        return unterminatedString(quoteChar, start, asciiOnly);
       }
       if (next > 127) asciiOnly = false;
       next = advance();
@@ -1089,14 +1090,14 @@ abstract class AbstractScanner implements Scanner {
         return next;
       } else if (identical(next, $LF) || identical(next, $CR)) {
         if (!asciiOnly) handleUnicode(start);
-        return unterminatedRawString(quoteChar);
+        return unterminatedRawString(quoteChar, start, asciiOnly);
       } else if (next > 127) {
         asciiOnly = false;
       }
       next = advance();
     }
     if (!asciiOnly) handleUnicode(start);
-    return unterminatedRawString(quoteChar);
+    return unterminatedRawString(quoteChar, start, asciiOnly);
   }
 
   int tokenizeMultiLineRawString(int quoteChar, int start) {
@@ -1134,7 +1135,7 @@ abstract class AbstractScanner implements Scanner {
       }
     }
     if (!asciiOnlyLine) handleUnicode(unicodeStart);
-    return unterminatedRawMultiLineString(quoteChar);
+    return unterminatedRawMultiLineString(quoteChar, start, asciiOnlyLine);
   }
 
   int tokenizeMultiLineString(int quoteChar, int start, bool raw) {
@@ -1185,7 +1186,7 @@ abstract class AbstractScanner implements Scanner {
       next = advance();
     }
     if (!asciiOnlyLine) handleUnicode(unicodeStart);
-    return unterminatedMultiLineString(quoteChar);
+    return unterminatedMultiLineString(quoteChar, start, asciiOnlyString);
   }
 
   int unexpected(int character) {
@@ -1198,22 +1199,40 @@ abstract class AbstractScanner implements Scanner {
     return advanceAfterError(shouldAdvance);
   }
 
-  int unterminatedString(int quoteChar) {
-    return unterminated(new String.fromCharCodes([quoteChar]));
+  void terminateUnterminatedString(
+      int start, bool asciiOnlyString, String suffix) {
+    if (start < scanOffset) {
+      appendSubstringToken(TokenType.STRING, start, asciiOnlyString);
+    }
+    beginToken();
+    appendToken(
+        new SyntheticStringToken(TokenType.STRING, suffix, tokenStart, 0));
+  }
+
+  int unterminatedString(int quoteChar, int start, bool asciiOnlyString) {
+    String suffix = new String.fromCharCodes([quoteChar]);
+    terminateUnterminatedString(start, asciiOnlyString, suffix);
+    return unterminated(suffix);
   }
 
-  int unterminatedRawString(int quoteChar) {
-    return unterminated('r${new String.fromCharCodes([quoteChar])}');
+  int unterminatedRawString(int quoteChar, int start, bool asciiOnlyString) {
+    String suffix = new String.fromCharCodes([quoteChar]);
+    terminateUnterminatedString(start, asciiOnlyString, suffix);
+    return unterminated('r$suffix');
   }
 
-  int unterminatedMultiLineString(int quoteChar) {
-    return unterminated(
-        new String.fromCharCodes([quoteChar, quoteChar, quoteChar]));
+  int unterminatedMultiLineString(
+      int quoteChar, int start, bool asciiOnlyString) {
+    String suffix = new String.fromCharCodes([quoteChar, quoteChar, quoteChar]);
+    terminateUnterminatedString(start, asciiOnlyString, suffix);
+    return unterminated(suffix);
   }
 
-  int unterminatedRawMultiLineString(int quoteChar) {
-    return unterminated(
-        'r${new String.fromCharCodes([quoteChar, quoteChar, quoteChar])}');
+  int unterminatedRawMultiLineString(
+      int quoteChar, int start, bool asciiOnlyString) {
+    String suffix = new String.fromCharCodes([quoteChar, quoteChar, quoteChar]);
+    terminateUnterminatedString(start, asciiOnlyString, suffix);
+    return unterminated('r$suffix');
   }
 
   int advanceAfterError(bool shouldAdvance) {