Revert earlier changes to fix bots (TBR)

pkg/analyzer/lib/src/fasta/ast_builder.dart

 // Copyright (c) 2016, 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.
 
 library fasta.analyzer.ast_builder;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast_factory.dart' show AstFactory;
 import 'package:analyzer/dart/ast/standard_ast_factory.dart' as standard;
 import 'package:analyzer/dart/ast/token.dart' show Token, TokenType;
@@ skipping 53 matching lines @@
   /// in class, method, and function declarations.
   ///
   /// This is being replaced by the @native(...) annotation.
   //
   // TODO(danrubel) Move this flag to a better location
   // and should only be true if either:
   // * The current library is a platform library
   // * The current library has an import that uses the scheme "dart-ext".
   bool allowNativeClause = false;
 
-  /// A flag indicating whether an exception should be thrown when an error is
-  /// reported for which we have no mapping into an analyzer error.
-  //
-  // TODO(brianwilkerson) Remove this flag after failing tests have been triaged.
-  bool throwOnMissingErrorMapping = true;
-
   AstBuilder(this.errorReporter, this.library, this.member, Scope scope,
       this.isFullAst,
       [Uri uri])
       : uri = uri ?? library.fileUri,
         super(scope);
 
   createJumpTarget(JumpTargetKind kind, int charOffset) {
     // TODO(ahe): Implement jump targets.
     return null;
   }
@@ skipping 990 matching lines @@
       String lexeme = token.lexeme;
       if (identical('async', lexeme) || identical('yield', lexeme)) {
         errorReporter?.reportErrorForOffset(
             ParserErrorCode.ASYNC_KEYWORD_USED_AS_IDENTIFIER,
             token.charOffset,
             token.charCount);
         push(ast.simpleIdentifier(token));
         return token;
       }
     }
-    if (throwOnMissingErrorMapping) {
-      throw new UnimplementedError('Failed to map $message at $token');
-    }
+    return super.handleUnrecoverableError(token, message);
   }
 
   void handleUnaryPrefixExpression(Token token) {
     debugEvent("UnaryPrefixExpression");
     push(ast.prefixExpression(token, pop()));
   }
 
   void handleUnaryPrefixAssignmentExpression(Token token) {
     debugEvent("UnaryPrefixAssignmentExpression");
     push(ast.prefixExpression(token, pop()));
@@ skipping 854 matching lines @@
           errorReporter?.reportErrorForOffset(
               ParserErrorCode.EXPECTED_TOKEN, charOffset, text.length, [text]);
         } else {
           errorReporter?.reportErrorForOffset(ParserErrorCode.UNEXPECTED_TOKEN,
               charOffset, text.length, [text]);
         }
         return;
       default:
       // fall through
     }
-    if (throwOnMissingErrorMapping) {
-      throw new UnimplementedError('Failed to map $message at $charOffset');
-    }
   }
 
   /// A marker method used to mark locations where a token is being located in
   /// an unsafe way. In all such cases the parser needs to be fixed to pass in
   /// the token.
   Token unsafeToken(Token token, TokenType tokenType) {
     // TODO(brianwilkerson) Eliminate the need for this method.
     return token.type == tokenType ? token : null;
   }
 }
@@ skipping 96 matching lines @@
       } else if (identical('var', s)) {
         finalConstOrVarKeyword = token;
       } else if (identical('covariant', s)) {
         covariantKeyword = token;
       } else {
         unhandled("$s", "modifier", token.charOffset, null);
       }
     }
   }
 }