Problems reported:

parser/extensions/const:1:1: Expected 'on' after this.
extension const Extension on int {}
^^^^^^^^^

parser/extensions/const:1:11: Expected a type, but got 'const'.
extension const Extension on int {}
          ^^^^^

parser/extensions/const:1:11: An extension declaration must have a body, even if it is empty.
extension const Extension on int {}
          ^^^^^

parser/extensions/const:1:27: The const variable 'on' must be initialized.
extension const Extension on int {}
                          ^^

parser/extensions/const:1:27: Expected ';' after this.
extension const Extension on int {}
                          ^^

parser/extensions/const:1:30: A function declaration needs an explicit list of parameters.
extension const Extension on int {}
                             ^^^

beginCompilationUnit(extension)
  beginMetadataStar(extension)
  endMetadataStar(0)
  beginExtensionDeclarationPrelude(extension)
    handleNoTypeVariables(const)
    beginExtensionDeclaration(null, extension, null)
      handleRecoverableError(Message[ExpectedAfterButGot, Expected 'on' after this., null, {string: on}], extension, extension)
      handleRecoverableError(Message[ExpectedType, Expected a type, but got 'const'., null, {lexeme: const}], const, const)
      handleIdentifier(, typeReference)
      handleNoTypeArguments(const)
      handleType(, null)
      handleRecoverableError(ExpectedExtensionBody, const, const)
      beginClassOrMixinOrExtensionBody(DeclarationKind.Extension, {)
      endClassOrMixinOrExtensionBody(DeclarationKind.Extension, 0, {, })
    endExtensionDeclaration(extension, extension, on, })
  endTopLevelDeclaration(})
  beginMetadataStar(const)
  endMetadataStar(0)
  beginTopLevelMember(const)
    beginFields(DeclarationKind.TopLevel, null, null, null, null, null, null, const, })
      handleIdentifier(Extension, typeReference)
      handleNoTypeArguments(on)
      handleType(Extension, null)
      handleIdentifier(on, topLevelVariableDeclaration)
      handleRecoverableError(Message[ConstFieldWithoutInitializer, The const variable 'on' must be initialized., Try adding an initializer ('= expression') to the declaration., {name: on}], on, on)
      handleNoFieldInitializer(int)
      handleRecoverableError(Message[ExpectedAfterButGot, Expected ';' after this., null, {string: ;}], on, on)
    endTopLevelFields(null, null, null, null, null, const, 1, const, ;)
  endTopLevelDeclaration(;)
  beginMetadataStar(int)
  endMetadataStar(0)
  beginTopLevelMember(int)
    beginTopLevelMethod(;, null, null)
      handleNoType(;)
      handleIdentifier(int, topLevelFunctionDeclaration)
      handleNoTypeVariables({)
      handleRecoverableError(MissingFunctionParameters, int, int)
      beginFormalParameters((, MemberKind.TopLevelMethod)
      endFormalParameters(0, (, ), MemberKind.TopLevelMethod)
      handleAsyncModifier(null, null)
      beginBlockFunctionBody({)
      endBlockFunctionBody(0, {, })
    endTopLevelMethod(int, null, })
  endTopLevelDeclaration(})
endCompilationUnit(3, )