[JS] When building source maps getting p.getLineCount is not a function

[arichiardi]

Hello Google folks!

First of all I am sorry if this is not the place for this bug report, it is concerning the JS port.
Note that I don't have a reproducible case with only the compiler, but I can make one in case it is needed.

When generating source maps using lumo, which uses google-closure-compiler-js, I get an error:

p.getLineCount is not a function
	 HJ (google-closure-compiler-js.cljs:7737:245)
	 l2c (google-closure-compiler-js.cljs:6713:84)
	 zHb (google-closure-compiler-js.cljs:5842:99)
	 P1c (google-closure-compiler-js.cljs:6145:152)
	 tDb (google-closure-compiler-js.cljs:5056:103)
	 iDb (google-closure-compiler-js.cljs:7117:315)
	 jDb (google-closure-compiler-js.cljs:5569:102)
	 yIb (google-closure-compiler-js.cljs:6208:201)
	 TIb (google-closure-compiler-js.cljs:7722:452)
	 (UIb.VIb)
	 jJb (google-closure-compiler-js.cljs:4846:36)
	 wIb (google-closure-compiler-js.cljs:1758:23)
	 NIb (google-closure-compiler-js.cljs:6165:148)
	 (OIb.PIb)
	 jJb (google-closure-compiler-js.cljs:4846:36)
	 sjd (google-closure-compiler-js.cljs:6538:35)
	 jjd (google-closure-compiler-js.cljs:7692:407)
	 hK (google-closure-compiler-js.cljs:1833:29)
	 kK (google-closure-compiler-js.cljs:3201:44)
	 (google-closure-compiler-js.cljs:3404:46)
	 compile (google-closure-compiler-js.cljs:8106:15)
	 Function.lumo.closure.optimize.cljs$core$IFn$_invoke$arity$variadic (evalmachine.<anonymous>:2623:123)
...

The steps are described here anmonteiro/lumo#361, but more importantly I was wondering whether the source map portion of the code is known to be stable.

Thanks a lot for your hard work!

[MatrixFrog]

It looks like we need to write an implementation at https://github.com/google/closure-compiler/blob/master/src/com/google/debugging/sourcemap/SourceMapObjectParserJs.java#L60

I assume just return this.lineCount; will do.

[arichiardi]

So this might be a very noob question but I got a compiler error saying that a native method cannot have an implementation. Additionally, it seems that .getLineCount is used only there being a private class. I might be wrong but maybe the intent there was to get rid of it?

Line 94 can become:

    builder.setLineCount(sourceMap.lineCount);

I don't know what happens when JS properties are undefined, maybe a check is appropriate there.

What do you think @MatrixFrog?

[arichiardi]

Just came across the syntax with */

Maybe:

    @JsMethod
    native Object getLineCount() /*-{ return this.lineCount; }-*/;

?

[arichiardi]

I confirm the proposed PR solves the issue.