Move parser helper classes to own files and clean them up.

pkg/front_end/lib/src/fasta/parser/assert.dart

Index: pkg/front_end/lib/src/fasta/parser/assert.dart
diff --git a/pkg/front_end/lib/src/fasta/parser/assert.dart b/pkg/front_end/lib/src/fasta/parser/assert.dart
new file mode 100644
index 0000000000000000000000000000000000000000..d6a97945a576153dd6720ff7529a328bd2dac5ac
--- /dev/null
+++ b/pkg/front_end/lib/src/fasta/parser/assert.dart
@@ -0,0 +1,16 @@
+// 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 license that can be found in the LICENSE file.
+
+library fasta.parser.Assert;
+
+/// Syntactic forms of `assert`.
+///
+/// An assertion can legally occur as a statement. However, assertions are also
+/// experimentally allowed in initializers. For improved error recovery, we
+/// also attempt to parse asserts as expressions.
+enum Assert {
+  Expression,
+  Initializer,
+  Statement,
+}