Fix some problems with lexical error recovery.

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

 // Copyright (c) 2017, 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 licenset hat can be found in the LICENSE file.
 
 library fasta.scanner.recover;
 
 import 'token.dart' show
     StringToken,
     Token;
 
@@ skipping 95 matching lines @@
         beforeGoodTail = null;
       } else {
         goodTail = beforeGoodTail;
       }
     }
     if (append) {
       codeUnits.addAll(next.value.codeUnits);
       next = next.next;
     }
     String value = new String.fromCharCodes(codeUnits);
-    Token recovered = synthesizeToken(
-        charOffset, value, Precedence.IDENTIFIER_INFO);
-    recovered.next = next;
-    return recovered;
+    return synthesizeToken(charOffset, value, Precedence.IDENTIFIER_INFO)
+        ..next = next;
   }
 
   recoverExponent() {
-    return synthesizeToken(errorTail.charOffset, "NaN", Precedence.DOUBLE_INFO);
+    return synthesizeToken(errorTail.charOffset, "NaN", Precedence.DOUBLE_INFO)
+        ..next = errorTail.next;
   }
 
   recoverString() {
     // TODO(ahe): Improve this.
     return skipToEof(errorTail);
   }
 
   recoverHexDigit() {
-    return synthesizeToken(errorTail.charOffset, "-1", Precedence.INT_INFO);
+    return synthesizeToken(errorTail.charOffset, "-1", Precedence.INT_INFO)
+        ..next = errorTail.next;
   }
 
   recoverStringInterpolation() {
     // TODO(ahe): Improve this.
     return skipToEof(errorTail);
   }
 
   recoverComment() {
     // TODO(ahe): Improve this.
     return skipToEof(errorTail);
   }
 
   recoverUnmatched() {
     // TODO(ahe): Try to use top-level keywords (such as `class`, `typedef`,
     // and `enum`) and identation to recover.
-    return errorTail;
+    return errorTail.next;
   }
 
   for (Token current = tokens; !current.isEof; current = current.next) {
     if (current is ErrorToken) {
       ErrorToken first = current;
       Token next = current;
       bool treatAsWhitespace = false;
       do {
         current = next;
         if (errorTail == null) {
           error = next;
         } else {
           errorTail.next = next;
         }
         errorTail = next;
         next = next.next;
       } while (next is ErrorToken && first.errorCode == next.errorCode);
 
       switch (first.errorCode) {
         case ErrorKind.Encoding:
         case ErrorKind.NonAsciiWhitespace:
         case ErrorKind.AsciiControlCharacter:
           treatAsWhitespace = true;
           break;
 
         case ErrorKind.NonAsciiIdentifier:
           current = recoverIdentifier(first);
+          assert(current.next != null);
           break;
 
         case ErrorKind.MissingExponent:
           current = recoverExponent();
+          assert(current.next != null);
           break;
 
         case ErrorKind.UnterminatedString:
           current = recoverString();
+          assert(current.next != null);
           break;
 
         case ErrorKind.ExpectedHexDigit:
           current = recoverHexDigit();
+          assert(current.next != null);
           break;
 
         case ErrorKind.UnexpectedDollarInString:
           current = recoverStringInterpolation();
+          assert(current.next != null);
           break;
 
         case ErrorKind.UnterminatedComment:
           current = recoverComment();
+          assert(current.next != null);
           break;
 
         case ErrorKind.UnmatchedToken:
           current = recoverUnmatched();
+          assert(current.next != null);
           break;
 
         case ErrorKind.UnterminatedToken: // TODO(ahe): Can this happen?
         default:
           treatAsWhitespace = true;
           break;
       }
       if (treatAsWhitespace) continue;
     }
     if (goodTail == null) {
@@ skipping 22 matching lines @@
 
 String closeBraceFor(String openBrace) {
   return const {
     '(': ')',
     '[': ']',
     '{': '}',
     '<': '>',
     r'${': '}',
   }[openBrace];
 }