[js] add a --source-map-debug flag to help with getting correcting so…

…urce maps

src/vm/js/Chunk.nqp

@@ -61,6 +61,25 @@ class Chunk {
         }
     }
 
+    method source_map_debug() {
+        my $js := ''; 
+        for @!setup -> $part {
+           if nqp::isstr($part) {
+              $js := $js ~ $part;
+           } else {
+              $js := $js ~ $part.source_map_debug;
+           }
+        }
+
+        if nqp::defined($!node) && $!node.node {
+            my $node := $!node.node;
+            my $line := HLL::Compiler.lineof($node.orig(), $node.from(), :cache(1));
+            "/* LINE $line */\n" ~ $js ~ "/* END $line */"
+        } else {
+            $js;
+        }
+    }
+
     method type() {
         $!type;
     }

src/vm/js/Compiler.nqp

@@ -1410,4 +1410,8 @@ class QAST::CompilerJS does DWIMYNameMangling does SerializeOnce {
     method emit_with_source_map($ast) {
        self.as_js_with_prelude($ast).with_source_map_info
     }
+
+    method emit_with_source_map_debug($ast) {
+       self.as_js_with_prelude($ast).source_map_debug
+    }
 }

src/vm/js/HLL/Backend.nqp

@@ -56,7 +56,9 @@ class HLLBackend::JavaScript {
         my $backend := QAST::CompilerJS.new(nyi=>%adverbs<nyi> // 'ignore', cps=>%adverbs<cps> // 'on');
 
 
-        if %adverbs<source-map> {
+        if %adverbs<source-map-debug> {
+            $backend.emit_with_source_map_debug($qast);
+        } elsif %adverbs<source-map> {
             $backend.emit_with_source_map($qast);
         } else {
             my $code := $backend.emit($qast);

src/vm/js/bin/nqp-js.nqp

@@ -13,6 +13,7 @@ sub MAIN(*@ARGS) {
 
     my @clo := $nqpcomp-cc.commandline_options();
     @clo.push('source-map');
+    @clo.push('source-map-debug');
     @clo.push('beautify');
     @clo.push('cps=s');
     @clo.push('nyi=s');