update syntax to use const, let and this

refactor execution context

 - remix-analyzer@0.3.19
 - remix-astwalker@0.0.15
 - remix-debug@0.3.21
 - remix-lib@0.4.18
 - remix-simulator@0.1.9-alpha.11
 - remix-solidity@0.3.21
 - remix-tests@0.1.24
 - remix-url-resolver@0.0.10

 - remix-analyzer@0.3.18
 - remix-astwalker@0.0.14
 - remix-debug@0.3.20
 - remix-lib@0.4.17
 - remix-simulator@0.1.9-alpha.10
 - remix-solidity@0.3.20
 - remix-tests@0.1.23
 - remix-url-resolver@0.0.9