improve source-map generation (#3782)

- emit singleton segments to mark generated code from input

lib/output.js

@@ -1470,7 +1470,6 @@ function OutputStream(options) {
         AST_Node,
         // since the label symbol will mark it
         AST_LabeledStatement,
-        AST_Toplevel,
     ], noop);
 
     // XXX: I'm not exactly sure if we need it for all of these nodes,
@@ -1482,7 +1481,6 @@ function OutputStream(options) {
         AST_Constant,
         AST_Debugger,
         AST_Definitions,
-        AST_Directive,
         AST_Finally,
         AST_Jump,
         AST_Lambda,

lib/sourcemap.js

@@ -119,6 +119,7 @@ function SourceMap(options) {
             if (content) sources_content[map.sources[i]] = content;
         }
     });
+    var prev_source;
     var generated_line = 1;
     var generated_column = 0;
     var source_index = 0;
@@ -137,11 +138,14 @@ function SourceMap(options) {
                     if (orig_col >= col) indices = segments[i];
                     if (orig_col <= col) break;
                 }
-                if (!indices || indices.length < 4) return;
-                source = map.sources[indices[1]];
-                orig_line = indices[2];
-                orig_col = indices[3];
-                if (indices.length > 4) name = map.names[indices[4]];
+                if (!indices || indices.length < 4) {
+                    source = null;
+                } else {
+                    source = map.sources[indices[1]];
+                    orig_line = indices[2];
+                    orig_col = indices[3];
+                    if (indices.length > 4) name = map.names[indices[4]];
+                }
             }
             add(source, gen_line, gen_col, orig_line, orig_col, name);
         } : add,
@@ -164,6 +168,8 @@ function SourceMap(options) {
     };
 
     function add(source, gen_line, gen_col, orig_line, orig_col, name) {
+        if (prev_source == null && source == null) return;
+        prev_source = source;
         if (generated_line < gen_line) {
             generated_column = 0;
             do {
@@ -174,6 +180,7 @@ function SourceMap(options) {
         }
         mappings += vlq_encode(gen_col - generated_column);
         generated_column = gen_col;
+        if (source == null) return;
         var src_idx = sources.index(source);
         mappings += vlq_encode(src_idx - source_index);
         source_index = src_idx;