improve fasta interpolation 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 '../../scanner/token.dart' show TokenType;
 
 import '../fasta_codes.dart'
     show
@@ skipping 141 matching lines @@
     return skipToEof(errorTail);
   }
 
   recoverUnmatched() {
     // TODO(ahe): Try to use top-level keywords (such as `class`, `typedef`,
     // and `enum`) and identation to recover.
     return errorTail.next;
   }
 
   for (Token current = tokens; !current.isEof; current = current.next) {
-    if (current is ErrorToken) {
+    while (current is ErrorToken) {
       ErrorToken first = current;
       Token next = current;
-      bool treatAsWhitespace = false;
       do {
         current = next;
         if (errorTail == null) {
           error = next;
         } else {
           errorTail.next = next;
           next.previous = errorTail;
         }
         errorTail = next;
         next = next.next;
       } while (next is ErrorToken && first.errorCode == next.errorCode);
 
       FastaCode code = first.errorCode;
       if (code == codeEncoding ||
           code == codeNonAsciiWhitespace ||
           code == codeAsciiControlCharacter) {
-        treatAsWhitespace = true;
+        current = errorTail.next;
       } else if (code == codeNonAsciiIdentifier) {
         current = recoverIdentifier(first);
         assert(current.next != null);
       } else if (code == codeMissingExponent) {
         current = recoverExponent();
         assert(current.next != null);
       } else if (code == codeUnterminatedString) {
         current = recoverString();
         assert(current.next != null);
       } else if (code == codeExpectedHexDigit) {
         current = recoverHexDigit();
         assert(current.next != null);
       } else if (code == codeUnexpectedDollarInString) {
         current = recoverStringInterpolation();
         assert(current.next != null);
       } else if (code == codeUnterminatedComment) {
         current = recoverComment();
         assert(current.next != null);
       } else if (code == codeUnmatchedToken) {
         current = recoverUnmatched();
         assert(current.next != null);
       } else {
-        treatAsWhitespace = true;
+        current = errorTail.next;
       }
-      if (treatAsWhitespace) continue;
     }
     if (goodTail == null) {
       good = current;
     } else {
       goodTail.next = current;
       current.previous = goodTail;
     }
     beforeGoodTail = goodTail;
     goodTail = current;
   }
@@ skipping 24 matching lines @@
 
 String closeBraceFor(String openBrace) {
   return const {
     '(': ')',
     '[': ']',
     '{': '}',
     '<': '>',
     r'${': '}',
   }[openBrace];
 }