enhance imports (#5287)

lib/compress.js

@@ -459,6 +459,7 @@ Compressor.prototype.compress = function(node) {
         if (ref.in_arg && is_funarg(def)) return false;
         return all(def.orig, function(sym) {
             if (sym instanceof AST_SymbolConst || sym instanceof AST_SymbolLet) {
+                if (sym instanceof AST_SymbolImport) return true;
                 return compressor && can_varify(compressor, sym);
             }
             return !(keep_lambda && sym instanceof AST_SymbolLambda);

test/compress/imports.js

@@ -106,6 +106,22 @@ drop_unused: {
     }
 }
 
+drop_side_effect_free: {
+    options = {
+        imports: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        import foo from "bar";
+        var a = foo;
+    }
+    expect: {
+        import "bar";
+    }
+}
+
 mangle: {
     rename = false
     mangle = {