Fix getBindingIdentifiers in babel-types

[rtsao]

Q	A
Patch: Bug Fix?	yes
Major: Breaking Change?	no
Minor: New Feature?	no
Deprecations?	no
Spec Compliancy?	yes
Tests Added/Pass?	yes
Fixed Tickets
License	MIT
Doc PR	no
Dependency Changes	no

Summary of changes:

• Added passing tests for t.getBindingIdentifiers (the first two tests)
• Added a test which should pass, but fails without the fix in this PR (the third test)
• Fixed logic in t.getBindingIdentifiers regarding handling of ExportDeclarations

The correct logic (which is added in this PR) is currently in reflected in the equivalent implementation of getBindingIdentifierPaths function in babel-traverse:

// …
    if (id.isExportDeclaration()) {
      const declaration = id.get("declaration");
      if (declaration.isDeclaration()) {
        search.push(declaration);
      }
      continue;
    }
// …

[codecov-io]

Current coverage is 89.15% (diff: 100%)

Merging #5068 into master will not change coverage

@@             master      #5068   diff @@
==========================================
  Files           203        203          
  Lines          9821       9821          
  Methods        1072       1072          
  Messages          0          0          
  Branches       2614       2614          
==========================================
  Hits           8756       8756          
  Misses         1065       1065          
  Partials          0          0          

Powered by Codecov. Last update 796c6c0...ae87097

[hzoo]

Awesome work @rtsao (I see it's your first PR too)! Yeah we need to figure out how to separate babel-types/traverse/core logic better

[hzoo]

Released in https://github.com/babel/babel/releases/tag/v6.22.0