Add unit tests

Author: kungfooman

test/typechecking/import-as-input.mjs

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+import * as React from 'react';

test/typechecking/import-as-output.mjs

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+import * as React from 'react';
+registerImportNamespaceSpecifier('React', React);

test/typechecking/import-both-input.mjs

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+import React, {useState, useEffect, useRef} from 'react';

test/typechecking/import-both-output.mjs

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+import React, {useState, useEffect, useRef} from 'react';

test/typechecking/import.mjs renamed to test/typechecking/import-runtime-input.mjs

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+const {
+  parse
+} = await import("@babel/parser");

test/typechecking/import-runtime-output.mjs

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+// This is only a Babel plugin:
+// const {parse} = await import("@babel/parser", {abc: 123});