Improve parsing of async and generator methods.

pkg/front_end/lib/src/fasta/parser/error_kind.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.parser.error_kind;
 
 /// Kinds of error codes.
 enum ErrorKind {
+  AbstractNotSync,
   AsciiControlCharacter,
+  AsyncAsIdentifier,
+  AwaitAsIdentifier,
+  AwaitNotAsync,
   BuiltInIdentifierAsType,
   BuiltInIdentifierInDeclaration,
   EmptyNamedParameterList,
   EmptyOptionalParameterList,
   Encoding,
   ExpectedBlockToSkip,
   ExpectedBody,
   ExpectedButGot,
   ExpectedClassBody,
 
   /// This error code can be used to support non-compliant (with respect to
   /// Dart Language Specification) Dart VM native clauses. See
   /// [dart_vm_native.dart].
   ExpectedClassBodyToSkip,
+  AwaitForNotAsync,
   ExpectedDeclaration,
   ExpectedExpression,
   ExpectedFunctionBody,
   ExpectedHexDigit,
   ExpectedIdentifier,
   ExpectedOpenParens,
   ExpectedString,
   ExpectedType,
   ExtraneousModifier,
   ExtraneousModifierReplace,
+  FactoryNotSync,
+  GeneratorReturnsValue,
+  InvalidAwaitFor,
   InvalidInlineFunctionType,
-  InvalidAwaitFor,
   InvalidSyncModifier,
   InvalidVoid,
   MissingExponent,
   NonAsciiIdentifier,
   NonAsciiWhitespace,
   PositionalParameterWithEquals,
   RequiredParameterWithDefault,
+  SetterNotSync,
   StackOverflow,
   UnexpectedDollarInString,
   UnexpectedToken,
   UnmatchedToken,
   UnsupportedPrefixPlus,
   UnterminatedComment,
   UnterminatedString,
   UnterminatedToken,
+  YieldAsIdentifier,
+  YieldNotGenerator,
   Unspecified,
 }