Document where the parser uses peeking.

pkg/front_end/lib/src/fasta/parser/parser.md

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+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.
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+
+# Uses of peek in the parser
+
+  * In parseTypedef, the parser uses peekAfterNominalType to select between old
+    style and new style.
+
+  * In parseClassOrNamedMixinApplication, the parser uses peekAfterNominalType
+    to select between named mixin application or class declaration.
+
+  * In parseClass, the parser uses peekAfterNominalType to select between a
+    mixin application and a regular extends.
+
+  * In parseType, the parser uses peekAfterIfType to tell the difference
+    between `id` and `id id`.
+
+  * In parseExpressionStatementOrDeclaration, the parser uses
+    peekIdentifierAfterType to select between an expression statement or a
+    local (function or variable) declaration.
+
+  * In parseExpressionStatementOrConstDeclaration, the parser uses
+    peekIdentifierAfterOptionalType to select between an expression statement
+    and a const local variable.
+
+  * In parseSendOrFunctionLiteral, the parser uses peekAfterIfType to select
+    between function expression and send.
+
+  * In parseVariablesDeclarationOrExpressionOpt, the parser uses
+    peekIdentifierAfterType to select between local variable declarations or an
+    expression.
+
+  * In parseSwitchCase, the parser uses peekPastLabels to select between case
+    labels and statement labels.
+
+  * The parser uses isGeneralizedFunctionType in parseType, and findMemberName.
+
+  * The parser uses findMemberName in parseTopLevelMember, and parseMember.