Ensure callback function is bound to the right scope

[DonJayamanne]

Fixes #2281

• Title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• [n/a] Unit tests & code coverage are not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)
• [n/a] Dependencies are pinned (e.g. "1.2.3", not "^1.2.3")
• [n/a] package-lock.json has been regenerated if dependencies have changed

[d3r3kk]

Looks good to me - the test seems to be failing for (probably) an unrelated problem:

#[error]out\test\unittests\argsService.unit.test.js(,): error : FAILED pytest :: Check for new/unrecognized options without values
2018-07-31T15:06:36.8020498Z ##[debug]Processed: ##vso[task.logissue type=error;sourcepath=D:\a\1\s\out\test\unittests\argsService.unit.test.js;]FAILED pytest :: Check for new/unrecognized options without values

[codecov]

Codecov Report

Merging #2282 into master will decrease coverage by 0.38%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2282      +/-   ##
==========================================
- Coverage   79.82%   79.44%   -0.39%     
==========================================
  Files         310      310              
  Lines       14367    14367              
  Branches     2549     2549              
==========================================
- Hits        11469    11414      -55     
- Misses       2886     2941      +55     
  Partials       12       12

Flag	Coverage Δ
#MacOS	?
#Windows	?

Impacted Files	Coverage Δ
src/client/activation/languageServer.ts	26.08% <0%> (ø)	⬆️
src/client/common/platform/registry.ts	40.42% <0%> (-51.07%)	⬇️
src/client/common/platform/pathUtils.ts	66.66% <0%> (-33.34%)	⬇️
src/client/common/utils.ts	57.89% <0%> (-18.43%)	⬇️
src/client/common/platform/serviceRegistry.ts	88.88% <0%> (-11.12%)	⬇️
src/client/common/platform/platformService.ts	84.21% <0%> (-5.27%)	⬇️
src/client/common/process/pythonProcess.ts	88.63% <0%> (-4.55%)	⬇️
src/client/interpreter/locators/index.ts	91.48% <0%> (-4.26%)	⬇️
src/client/refactor/proxy.ts	81.05% <0%> (-3.16%)	⬇️
...interpreter/locators/services/KnownPathsService.ts	92.68% <0%> (-2.44%)	⬇️
... and 10 more

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