Thanks for the PR, @sosukesuzuki!

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	nx test @typescript-eslint/eslint-plugin-internal
	nx test @typescript-eslint/eslint-plugin-tslint
	nx test @typescript-eslint/eslint-plugin
	nx test @typescript-eslint/eslint-plugin-tslint
	nx test @typescript-eslint/eslint-plugin
	nx test @typescript-eslint/eslint-plugin-tslint
	nx test @typescript-eslint/eslint-plugin
	nx test @typescript-eslint/parser
	nx test @typescript-eslint/parser
	nx test @typescript-eslint/experimental-utils
	nx test @typescript-eslint/experimental-utils
	nx test @typescript-eslint/scope-manager
	nx test @typescript-eslint/scope-manager
	nx test @typescript-eslint/parser
	nx test @typescript-eslint/experimental-utils
	nx test @typescript-eslint/scope-manager
	nx test @typescript-eslint/visitor-keys
	nx test @typescript-eslint/typescript-estree
	nx test @typescript-eslint/visitor-keys
	nx test @typescript-eslint/typescript-estree

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This is actually currently done on purpose.
The TS parser is incredibly forgiving - and so it gives us a bad AST.
We don't currently enforce the AST matches the ESTree structure - so in this case import foo from bar is valid code in our parser (as is export { foo } from 1 + 1.

We have two options:

1. wait until we've done Parsing: strictly enforce the produced AST matches the spec and enforce most "error recovery" parsing errors #1852 and thus have parser errors for bad code
2. add validation in this diff to enforce that it is a string literal.

I'm happy with either - depends on if you've got time to do the extra work for (2).

@bradzacher Thanks you for telling me.

I'm happy with either - depends on if you've got time to do the extra work for (2).

I can work for implement option (2).

However, one question. Why source of ImportDeclaration is Literal? We can specify any expression to source of ImportDeclaration same as ExportNamedDeclaration.

I've pushed commits to add assertion for module specifiers, but I'm not sure it is correct way.

Codecov Report

Merging #3763 (5b66c4c) into v5 (a752638) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##               v5    #3763      +/-   ##
==========================================
- Coverage   92.68%   92.68%   -0.01%     
==========================================
  Files         327      325       -2     
  Lines       11351    11349       -2     
  Branches     3201     3203       +2     
==========================================
- Hits        10521    10519       -2     
  Misses        369      369              
  Partials      461      461              

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