Don't peek when parsing mixin applications.

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

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 # Uses of peek in the parser
 
-  * 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.