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Graal.js importing module causes org.graalvm.polyglot.PolyglotException: SyntaxError: Expected an operand but found import

I am trying to use experimental ES module support from Grall.js. I use following scripts: ES module "lib"

export const sqrt = Math.sqrt;
export function square(x) {
    return x * x;
export function diag(x, y) {
    return sqrt(square(x) + square(y));
and main script "script"
import { square, diag } from 'lib';
console.log('square(11)=' + square(11));
console.log('diag(4,3)=' + diag(4, 3));
I use graalvm-ce-19.2.1 and I use Polyglot inside JVM via JSR 223 to run the main script. 
Instead of trying to load lib from some place on disk it throws:
javax.script.ScriptException: org.graalvm.polyglot.PolyglotException: SyntaxError: script:1:0 Expected an operand but found import
import { square, diag } from 'lib';
script:1:30 Expected ; but found lib
import { square, diag } from 'lib';
        at com.oracle.truffle.js.scriptengine.GraalJSScriptEngine.eval(GraalJSScriptEngine.java:348)
        at com.oracle.truffle.js.scriptengine.GraalJSScriptEngine.eval(GraalJSScriptEngine.java:323)
What's wrong?
There is a convention on file names which trigger treating them as ES modules - file must end with .mjs. Alternatively it is possible to use (unofficial) mime type application/javascript+module on org.graalvm.polyglot.Source object. Without this import statement is not allowed 1. 
This is how it looks when using Polyglot Source/Context API:
String script = "import {x} from 'lib'";
// these two support "import"
Source source1 =  Source.newBuilder("js", script, "script.mjs").build();
Source source2 =  Source.newBuilder("js", script, "script").mimeType("application/javascript+module").build();
// this one doesn't
//Source source3 =  Source.newBuilder("js", script, "script").build(); 
and this is for JSR-223 API:
javax.script.ScriptEngine engine = factory.getEngineByName("graal.js");
engine.getContext().setAttribute(ScriptEngine.FILENAME, "script.mjs", ScriptContext.ENGINE_SCOPE);
It seems that there also was an older convention - using module: prefix, but this doesn't seem to work anymore.
                As far as I remember there is only this comment: github.com/graalvm/graaljs/issues/92#issuecomment-474858574
– tporeba
                Jul 28, 2020 at 8:32
                Oh it seems there is some new piece of documentation here github.com/graalvm/graaljs/blob/master/docs/user/…
– tporeba
                Jul 28, 2020 at 8:34
        
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