write sourceMappingURL custom section

src/de/inetsoftware/jwebassembly/binary/BinaryModuleWriter.java

@@ -125,6 +125,7 @@ public class BinaryModuleWriter extends ModuleWriter implements InstructionOpcod
         writeExportSection();
         writeCodeSection();
         writeDebugNames();
+        writeSourceMappingUrl();
         writeProducersSection();
 
         wasm.close();
@@ -272,6 +273,26 @@ public class BinaryModuleWriter extends ModuleWriter implements InstructionOpcod
         wasm.writeSection( SectionType.Custom, stream );
     }
 
+    /**
+     * Write the source mapping url
+     * 
+     * @throws IOException
+     *             if any I/O error occur
+     */
+    private void writeSourceMappingUrl() throws IOException {
+        if( !createSourceMap ) {
+            return;
+        }
+        String url = target.getSourceMappingURL();
+        if( url == null ) {
+            return;
+        }
+        WasmOutputStream stream = new WasmOutputStream();
+        stream.writeString( "sourceMappingURL" ); // Custom Section name "sourceMappingURL", content is part of the section length
+        stream.writeString( url );
+        wasm.writeSection( SectionType.Custom, stream );
+    }
+
     /**
      * Write producer information to wasm
      * 

src/de/inetsoftware/jwebassembly/module/WasmTarget.java

@@ -73,6 +73,19 @@ public class WasmTarget implements Closeable {
         return output;
     }
 
+    /**
+     * Get the URL for the source mapping that should be write into the assembly.
+     * 
+     * @return the URL string or null.
+     */
+    @Nonnull
+    public String getSourceMappingURL() {
+        if( file != null ) {
+            return file.getName() + ".map";
+        }
+        return null;
+    }
+
     /**
      * Get the source map OutputStream
      *